Optimize double alpha blending for 1/0 textures.

If the blend is 2*a / 1-a, and the alpha is either 1 or 0, we can still
use color doubling instead.  Fixes #3379 (Persona 2.)
Merge pull request #6072 from unknownbrackets/gpu-minor

Optimize double alpha blending for 1/0 textures