Auto merge of #16904 - paulrouget:notHeadLess, r=metajack
Auto merge of #16904 - paulrouget:notHeadLess, r=metajack

Not headless by default

Without this, initializing Servo without using the command line (when embedded) requires to explicitly set `opts.headless = false`, which is weird as it's the only option that needs to be changed.

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cbrewster/servo,canaltinova/servo,szeged/servo,saneyuki/servo,nnethercote/servo,emilio/servo,canaltinova/servo,SimonSapin/servo,thiagopnts/servo,dati91/servo,anthgur/servo,DominoTree/servo,cbrewster/servo,jimberlage/servo,DominoTree/servo,pyfisch/servo,jimberlage/servo,peterjoel/servo,thiagopnts/servo,CJ8664/servo,emilio/servo,avadacatavra/servo,CJ8664/servo,splav/servo,mbrubeck/servo,upsuper/servo,thiagopnts/servo,dsandeephegde/servo,ConnorGBrewster/servo,splav/servo,dati91/servo,dati91/servo,upsuper/servo,anthgur/servo,eddyb/servo,ConnorGBrewster/servo,DominoTree/servo,KiChjang/servo,DominoTree/servo,anthgur/servo,dsandeephegde/servo,SimonSapin/servo,thiagopnts/servo,peterjoel/servo,peterjoel/servo,fiji-flo/servo,mattnenterprise/servo,peterjoel/servo,KiChjang/servo,larsbergstrom/servo,larsbergstrom/servo,sadmansk/servo,mbrubeck/servo,ConnorGBrewster/servo,emilio/servo,pyfisch/servo,KiChjang/servo,saneyuki/servo,paulrouget/servo,notriddle/servo,canaltinova/servo,avadacatavra/servo,KiChjang/servo,ConnorGBrewster/servo,mbrubeck/servo,nnethercote/servo,paulrouget/servo,mattnenterprise/servo,mattnenterprise/servo,fiji-flo/servo,avadacatavra/servo,pyfisch/servo,nnethercote/servo,eddyb/servo,DominoTree/servo,rnestler/servo,eddyb/servo,splav/servo,pyfisch/servo,paulrouget/servo,mbrubeck/servo,eddyb/servo,thiagopnts/servo,notriddle/servo,dati91/servo,saneyuki/servo,KiChjang/servo,SimonSapin/servo,sadmansk/servo,fiji-flo/servo,dsandeephegde/servo,CJ8664/servo,canaltinova/servo,emilio/servo,dsandeephegde/servo,nnethercote/servo,dati91/servo,paulrouget/servo,dati91/servo,rnestler/servo,CJ8664/servo,mattnenterprise/servo,splav/servo,paulrouget/servo,fiji-flo/servo,DominoTree/servo,jimberlage/servo,SimonSapin/servo,peterjoel/servo,szeged/servo,avadacatavra/servo,saneyuki/servo,KiChjang/servo,saneyuki/servo,mattnenterprise/servo,peterjoel/servo,peterjoel/servo,DominoTree/servo,notriddle/servo,upsuper/servo,pyfisch/servo,saneyuki/servo,larsbergstrom/servo,thiagopnts/servo,szeged/servo,sadmansk/servo,notriddle/servo,SimonSapin/servo,rnestler/servo,eddyb/servo,paulrouget/servo,sadmansk/servo,CJ8664/servo,peterjoel/servo,rnestler/servo,jimberlage/servo,paulrouget/servo,paulrouget/servo,emilio/servo,SimonSapin/servo,larsbergstrom/servo,paulrouget/servo,anthgur/servo,rnestler/servo,fiji-flo/servo,nnethercote/servo,upsuper/servo,SimonSapin/servo,nnethercote/servo,paulrouget/servo,larsbergstrom/servo,eddyb/servo,canaltinova/servo,emilio/servo,avadacatavra/servo,sadmansk/servo,avadacatavra/servo,anthgur/servo,ConnorGBrewster/servo,ConnorGBrewster/servo,jimberlage/servo,mbrubeck/servo,upsuper/servo,KiChjang/servo,splav/servo,larsbergstrom/servo,eddyb/servo,notriddle/servo,upsuper/servo,pyfisch/servo,rnestler/servo,notriddle/servo,DominoTree/servo,larsbergstrom/servo,pyfisch/servo,saneyuki/servo,larsbergstrom/servo,DominoTree/servo,canaltinova/servo,sadmansk/servo,saneyuki/servo,anthgur/servo,cbrewster/servo,jimberlage/servo,mbrubeck/servo,jimberlage/servo,fiji-flo/servo,emilio/servo,upsuper/servo,szeged/servo,emilio/servo,cbrewster/servo,nnethercote/servo,cbrewster/servo,cbrewster/servo,ConnorGBrewster/servo,dsandeephegde/servo,SimonSapin/servo,larsbergstrom/servo,mattnenterprise/servo,cbrewster/servo,avadacatavra/servo,KiChjang/servo,szeged/servo,sadmansk/servo,CJ8664/servo,CJ8664/servo,notriddle/servo,jimberlage/servo,ConnorGBrewster/servo,canaltinova/servo,dati91/servo,mattnenterprise/servo,pyfisch/servo,jimberlage/servo,dsandeephegde/servo,emilio/servo,dsandeephegde/servo,szeged/servo,saneyuki/servo,splav/servo,pyfisch/servo,notriddle/servo,cbrewster/servo,DominoTree/servo,thiagopnts/servo,thiagopnts/servo,szeged/servo,peterjoel/servo,anthgur/servo,upsuper/servo,fiji-flo/servo,nnethercote/servo,fiji-flo/servo,nnethercote/servo,nnethercote/servo,szeged/servo,szeged/servo,pyfisch/servo,avadacatavra/servo,notriddle/servo,dati91/servo,splav/servo,KiChjang/servo,peterjoel/servo,saneyuki/servo,CJ8664/servo,splav/servo,larsbergstrom/servo,KiChjang/servo,eddyb/servo,mbrubeck/servo,sadmansk/servo,rnestler/servo,mattnenterprise/servo,szeged/servo,dsandeephegde/servo,mbrubeck/servo,splav/servo,notriddle/servo,splav/servo,anthgur/servo,rnestler/servo,canaltinova/servo,emilio/servo