GL: Added default back buffer FBO.
GL: Added default back buffer FBO.

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