Support fixed/fixed blending without a blit.
Convert blending to doubling in more cases.

We're not doubling the alpha, so all these cases are safe regardless of
the alpha value.
Support more src alpha doubling modes.

These prevent the need for more blits.
Reorganize alphablend state mapping logic.
Use shader blend for min/max when unsupported.
Merge pull request #6350 from unknownbrackets/gpu-blend

Avoid blitting for a bunch of gpu blending modes