Auto merge of #7186 - glennw:rounded-clip, r=pcwalton
Auto merge of #7186 - glennw:rounded-clip, r=pcwalton

Enable item clipping on normal transform layers. Fixes rounded corners on layers with 2d transforms.

Needed for #6643.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7186)
<!-- Reviewable:end -->

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