Auto merge of #13502 - servo:osmesa-debug, r=Ms2ger
Auto merge of #13502 - servo:osmesa-debug, r=Ms2ger

Fix OS-Mesa discovery in debug mode.

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