st: when creating an on-demand front color buffer, init to back buffer image
st: when creating an on-demand front color buffer, init to back buffer image

When we create a new front color buffer (user called glDrawBuffer(GL_FRONT))
initialize it to the contents of the back buffer.  Any previous call to
SwapBuffers() would have done that in effect, so make it reality.

mapbox/glsl-optimizer,KTXSoftware/glsl2agal,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,zeux/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,adobe/glsl2agal,KTXSoftware/glsl2agal,wolf96/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,metora/MesaGLSLCompiler,zz85/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,adobe/glsl2agal,tokyovigilante/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,metora/MesaGLSLCompiler,wolf96/glsl-optimizer,KTXSoftware/glsl2agal,zz85/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,KTXSoftware/glsl2agal,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,KTXSoftware/glsl2agal,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,zeux/glsl-optimizer,adobe/glsl2agal,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,adobe/glsl2agal,zeux/glsl-optimizer,jbarczak/glsl-optimizer,adobe/glsl2agal,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer,mapbox/glsl-optimizer,metora/MesaGLSLCompiler,zz85/glsl-optimizer,zeux/glsl-optimizer,mcanthony/glsl-optimizer,wolf96/glsl-optimizer,mapbox/glsl-optimizer,djreep81/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,tokyovigilante/glsl-optimizer