st/vega: Blending should use premultiplied alpha.
st/vega: Blending should use premultiplied alpha.

Convert color values to and back from premultiplied form for blending.
Finally the rendering result of the blend demo looks much closer to that
of the reference implementation.

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