[gpu] blend optimizations, handle more xfer modes with partial pixel coverage

[bsalomon, 2011-10-11 20:02:54]

With this change we can draw all the Porter-Duff modes correctly when the source
has partial pixel coverage and solid alpha. When the source has fractional alpha
there are 6 modes that force us to choose either partial coverage or correct
blending (unless we have dual source blending).

I didn't consider optimizations when the dst alpha is known to be zero or one or
when the src alpha is known to be zero. I also didn't fully optimize detecting
solid alpha when there is a color filter.

[reed1, 2011-10-11 21:20:04]

oiy, I have no idea
LGTM

[bsalomon, 2011-10-12 18:13:58]

Closed with r2464