Handle doubled alpha blending using premultiplication

This more accurately reproduces the effect, whether the color is white and the alpha half, or the color is half and the alpha is full.

It may have a performance impact, but I think some GPUs (?) append blending multiplications to the shader anyway, so it might not be too bad.  However, for more obscure blending modes it will definitely have a performance impact.  Unfortunately, it may still not draw correctly due to overlapping polygons.

Should fix #6550.

-[Unknown]