Auto merge of #7183 - glennw:scroll-clipping, r=pcwalton
Auto merge of #7183 - glennw:scroll-clipping, r=pcwalton

Add fix / hack for compositor repainting tiles on scroll layers that have stale clipping results.

Needed for #6643. Fixes #7153.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7183)
<!-- Reviewable:end -->

sadmansk/servo,j3parker/servo,rentongzhang/servo,avadacatavra/servo,caldwell/servo,cbrewster/servo,CJ8664/servo,wartman4404/servo,anthgur/servo,huonw/servo,cbrewster/servo,mattnenterprise/servo,sadmansk/servo,DominoTree/servo,A-deLuna/servo,tafia/servo,paulrouget/servo,dvberkel/servo,emilio/servo,larsbergstrom/servo,saratang/servo,steveklabnik/servo,kindersung/servo,nrc/servo,GyrosOfWar/servo,dati91/servo,jlegendary/servo,GyrosOfWar/servo,GyrosOfWar/servo,SimonSapin/servo,larsbergstrom/servo,meh/servo,mdibaiee/servo,steveklabnik/servo,splav/servo,canaltinova/servo,saneyuki/servo,peterjoel/servo,caldwell/servo,tafia/servo,nnethercote/servo,nnethercote/servo,thiagopnts/servo,thiagopnts/servo,WriterOfAlicrow/servo,kindersung/servo,cbrewster/servo,kindersung/servo,A-deLuna/servo,notriddle/servo,huonw/servo,DominoTree/servo,saneyuki/servo,avadacatavra/servo,sadmansk/servo,anthgur/servo,aidanhs/servo,avadacatavra/servo,saneyuki/servo,cbrewster/servo,walac/servo,dati91/servo,mbrubeck/servo,jimberlage/servo,eddyb/servo,cbrewster/servo,runarberg/servo,canaltinova/servo,pyfisch/servo,tafia/servo,tafia/servo,nnethercote/servo,rnestler/servo,mbrubeck/servo,jlegendary/servo,KiChjang/servo,peterjoel/servo,wartman4404/servo,ConnorGBrewster/servo,AnthonyBroadCrawford/servo,DominoTree/servo,eddyb/servo,dagnir/servo,akosel/servo,SimonSapin/servo,eddyb/servo,huonw/servo,A-deLuna/servo,dhananjay92/servo,nrc/servo,caldwell/servo,pyfisch/servo,brendandahl/servo,walac/servo,bjwbell/servo,bjwbell/servo,CJ8664/servo,j3parker/servo,bjwbell/servo,larsbergstrom/servo,mbrubeck/servo,DominoTree/servo,CJ8664/servo,RenaudParis/servo,splav/servo,WriterOfAlicrow/servo,anthgur/servo,CJ8664/servo,paulrouget/servo,upsuper/servo,aidanhs/servo,j3parker/servo,ConnorGBrewster/servo,jdramani/servo,nnethercote/servo,splav/servo,KiChjang/servo,larsbergstrom/servo,avadacatavra/servo,emilio/servo,mdibaiee/servo,notriddle/servo,meh/servo,anthgur/servo,dhananjay92/servo,DominoTree/servo,GyrosOfWar/servo,DominoTree/servo,jdramani/servo,dagnir/servo,pyfisch/servo,sadmansk/servo,rnestler/servo,mdibaiee/servo,dagnir/servo,brendandahl/servo,meh/servo,aidanhs/servo,akosel/servo,AnthonyBroadCrawford/servo,RenaudParis/servo,emilio/servo,avadacatavra/servo,aidanhs/servo,larsbergstrom/servo,steveklabnik/servo,thiagopnts/servo,splav/servo,nnethercote/servo,szeged/servo,thiagopnts/servo,anthgur/servo,dsandeephegde/servo,nnethercote/servo,CJ8664/servo,rentongzhang/servo,vks/servo,pyfisch/servo,youprofit/servo,avadacatavra/servo,jlegendary/servo,dvberkel/servo,rentongzhang/servo,upsuper/servo,anthgur/servo,jimberlage/servo,dati91/servo,sadmansk/servo,peterjoel/servo,tafia/servo,splav/servo,A-deLuna/servo,mattnenterprise/servo,steveklabnik/servo,brendandahl/servo,mdibaiee/servo,aidanhs/servo,saratang/servo,dagnir/servo,runarberg/servo,steveklabnik/servo,mattnenterprise/servo,paulrouget/servo,szeged/servo,peterjoel/servo,AnthonyBroadCrawford/servo,KiChjang/servo,peterjoel/servo,splav/servo,pyfisch/servo,AnthonyBroadCrawford/servo,rentongzhang/servo,fiji-flo/servo,runarberg/servo,WriterOfAlicrow/servo,RenaudParis/servo,peterjoel/servo,nrc/servo,rentongzhang/servo,fiji-flo/servo,paulrouget/servo,dati91/servo,mdibaiee/servo,dsandeephegde/servo,splav/servo,emilio/servo,ConnorGBrewster/servo,dvberkel/servo,rentongzhang/servo,wartman4404/servo,DominoTree/servo,wartman4404/servo,szeged/servo,bjwbell/servo,fiji-flo/servo,kindersung/servo,RenaudParis/servo,wartman4404/servo,szeged/servo,emilio/servo,youprofit/servo,saneyuki/servo,A-deLuna/servo,caldwell/servo,KiChjang/servo,szeged/servo,peterjoel/servo,SimonSapin/servo,akosel/servo,ConnorGBrewster/servo,mattnenterprise/servo,paulrouget/servo,huonw/servo,rnestler/servo,mbrubeck/servo,dhananjay92/servo,mbrubeck/servo,rnestler/servo,meh/servo,GyrosOfWar/servo,jdramani/servo,vks/servo,saratang/servo,huonw/servo,meh/servo,saneyuki/servo,GyrosOfWar/servo,paulrouget/servo,jdramani/servo,akosel/servo,nnethercote/servo,jdramani/servo,mattnenterprise/servo,codemac/servo,KiChjang/servo,jdramani/servo,codemac/servo,pyfisch/servo,youprofit/servo,dati91/servo,A-deLuna/servo,emilio/servo,KiChjang/servo,AnthonyBroadCrawford/servo,canaltinova/servo,SimonSapin/servo,walac/servo,notriddle/servo,splav/servo,RenaudParis/servo,kindersung/servo,szeged/servo,DominoTree/servo,j3parker/servo,pyfisch/servo,WriterOfAlicrow/servo,notriddle/servo,mbrubeck/servo,mdibaiee/servo,dsandeephegde/servo,splav/servo,dati91/servo,notriddle/servo,DominoTree/servo,upsuper/servo,canaltinova/servo,steveklabnik/servo,WriterOfAlicrow/servo,j3parker/servo,larsbergstrom/servo,notriddle/servo,canaltinova/servo,sadmansk/servo,huonw/servo,saneyuki/servo,nrc/servo,saratang/servo,jlegendary/servo,paulrouget/servo,jlegendary/servo,vks/servo,mdibaiee/servo,dati91/servo,eddyb/servo,dhananjay92/servo,codemac/servo,paulrouget/servo,mbrubeck/servo,runarberg/servo,fiji-flo/servo,vks/servo,CJ8664/servo,GyrosOfWar/servo,emilio/servo,KiChjang/servo,pyfisch/servo,fiji-flo/servo,meh/servo,j3parker/servo,caldwell/servo,jlegendary/servo,aidanhs/servo,saratang/servo,codemac/servo,saneyuki/servo,saratang/servo,KiChjang/servo,brendandahl/servo,runarberg/servo,paulrouget/servo,upsuper/servo,jdramani/servo,mbrubeck/servo,walac/servo,kindersung/servo,anthgur/servo,CJ8664/servo,emilio/servo,rnestler/servo,fiji-flo/servo,ConnorGBrewster/servo,dhananjay92/servo,tafia/servo,saneyuki/servo,RenaudParis/servo,szeged/servo,codemac/servo,fiji-flo/servo,larsbergstrom/servo,j3parker/servo,jimberlage/servo,vks/servo,anthgur/servo,saneyuki/servo,notriddle/servo,larsbergstrom/servo,upsuper/servo,steveklabnik/servo,dagnir/servo,eddyb/servo,A-deLuna/servo,dagnir/servo,KiChjang/servo,nnethercote/servo,peterjoel/servo,youprofit/servo,saneyuki/servo,rentongzhang/servo,dati91/servo,dhananjay92/servo,mattnenterprise/servo,WriterOfAlicrow/servo,nnethercote/servo,upsuper/servo,canaltinova/servo,dsandeephegde/servo,dvberkel/servo,peterjoel/servo,mattnenterprise/servo,mattnenterprise/servo,rnestler/servo,dsandeephegde/servo,runarberg/servo,ConnorGBrewster/servo,eddyb/servo,huonw/servo,nrc/servo,jimberlage/servo,fiji-flo/servo,cbrewster/servo,akosel/servo,SimonSapin/servo,szeged/servo,dsandeephegde/servo,caldwell/servo,brendandahl/servo,AnthonyBroadCrawford/servo,saratang/servo,avadacatavra/servo,rnestler/servo,dsandeephegde/servo,dsandeephegde/servo,KiChjang/servo,peterjoel/servo,larsbergstrom/servo,tafia/servo,splav/servo,SimonSapin/servo,RenaudParis/servo,eddyb/servo,notriddle/servo,youprofit/servo,sadmansk/servo,vks/servo,bjwbell/servo,runarberg/servo,SimonSapin/servo,upsuper/servo,rnestler/servo,dvberkel/servo,kindersung/servo,youprofit/servo,bjwbell/servo,dhananjay92/servo,jimberlage/servo,jimberlage/servo,vks/servo,thiagopnts/servo,ConnorGBrewster/servo,canaltinova/servo,akosel/servo,notriddle/servo,jlegendary/servo,emilio/servo,walac/servo,nrc/servo,SimonSapin/servo,walac/servo,szeged/servo,thiagopnts/servo,DominoTree/servo,akosel/servo,notriddle/servo,wartman4404/servo,pyfisch/servo,larsbergstrom/servo,nrc/servo,pyfisch/servo,eddyb/servo,jimberlage/servo,avadacatavra/servo,walac/servo,cbrewster/servo,emilio/servo,codemac/servo,canaltinova/servo,dagnir/servo,sadmansk/servo,thiagopnts/servo,AnthonyBroadCrawford/servo,dvberkel/servo,caldwell/servo,aidanhs/servo,dvberkel/servo,codemac/servo,WriterOfAlicrow/servo,upsuper/servo,jimberlage/servo,wartman4404/servo,thiagopnts/servo,brendandahl/servo,paulrouget/servo,cbrewster/servo,jimberlage/servo,szeged/servo,nnethercote/servo,bjwbell/servo,youprofit/servo,ConnorGBrewster/servo,meh/servo,CJ8664/servo,brendandahl/servo