Added GL backbuffer MSAA.
Added GL backbuffer MSAA.

LWJGL-CI/bgfx,darkimage/bgfx,janstk/bgfx,aonorin/bgfx,darkimage/bgfx,cyndis/bgfx,sergeScherbakov/bgfx,BlueCrystalLabs/bgfx,Vertexwahn/bgfx,Synxis/bgfx,Extrawurst/bgfx,BlueCrystalLabs/bgfx,jpcy/bgfx,kondrak/bgfx,andr3wmac/bgfx,0-wiz-0/bgfx,v3n/bgfx,Extrawurst/bgfx,emoon/bgfx,mendsley/bgfx,ocornut/bgfx,attilaz/bgfx,marco-we/bgfx,ktotheoz/bgfx,MikePopoloski/bgfx,mcanthony/bgfx,ming4883/bgfx,ocornut/bgfx,sergeScherbakov/bgfx,0-wiz-0/bgfx,ktotheoz/bgfx,bkaradzic/bgfx,ming4883/bgfx,janstk/bgfx,jpcy/bgfx,bkaradzic/bgfx,fluffyfreak/bgfx,jpcy/bgfx,mendsley/bgfx,andr3wmac/bgfx,BlueCrystalLabs/bgfx,ktotheoz/bgfx,MikePopoloski/bgfx,elmindreda/bgfx,jdryg/bgfx,LWJGL-CI/bgfx,Extrawurst/bgfx,0-wiz-0/bgfx,jdryg/bgfx,aonorin/bgfx,darkimage/bgfx,septag/bgfx,v3n/bgfx,jdryg/bgfx,emoon/bgfx,MikePopoloski/bgfx,cyndis/bgfx,jdryg/bgfx,andr3wmac/bgfx,mcanthony/bgfx,emoon/bgfx,septag/bgfx,LWJGL-CI/bgfx,ming4883/bgfx,fluffyfreak/bgfx,janstk/bgfx,jpcy/bgfx,LSBOSS/bgfx,mmicko/bgfx,aonorin/bgfx,Vertexwahn/bgfx,septag/bgfx,bkaradzic/bgfx,Synxis/bgfx,v3n/bgfx,cuavas/bgfx,marco-we/bgfx,mmicko/bgfx,LSBOSS/bgfx,kondrak/bgfx,elmindreda/bgfx,mmicko/bgfx,bkaradzic/bgfx,cyndis/bgfx,Vertexwahn/bgfx,LWJGL-CI/bgfx,ocornut/bgfx,cuavas/bgfx,mendsley/bgfx,kondrak/bgfx,attilaz/bgfx,Synxis/bgfx,fluffyfreak/bgfx,LSBOSS/bgfx,elmindreda/bgfx,attilaz/bgfx,fluffyfreak/bgfx,mcanthony/bgfx,cuavas/bgfx,marco-we/bgfx,sergeScherbakov/bgfx