Approximate better with two fixed blend colors.

Having one be constant, or having a nearby blend, is better than fixing
both to 1.0 or 0.0.
Merge pull request #1618 from unknownbrackets/gpu-minor

Approximate better with two fixed blend colors
I'm sorry. Comment was unnecessary
and sceAtrac too.
Merge branch 'master' of https://github.com/hrydgard/ppsspp into jpnfont