Apply unsupported blending modes in the shader.

This handles unsupported fixed color combinations, alpha doubling, etc.
Where possible, tries to avoid it - it means using a blit (without the
framebuffer fetch extension), which can slow things down a lot with tons
of drawcalls.
Double using multiply not add, should be fine.
Skip a few more fixa/fixb combos.
Log when blitting a ton per frame, it can get slow.
Remove outdated comment.

Was all lies.
Oops, typo.
Set a bit to account for fixa/fixb/etc. logic.
Unbind the fbo texture when it's not used.

Just in case there are problems with decimating it or etc.
Ignore the funcs for min/max/absdiff.

Seems from reports that they are indeed ignored, just wanted to verify.
Disable the scissor test around blits.

Because it applies.
Make sure to rebind a texture after changing it.
Merge pull request #6070 from unknownbrackets/gpu-blend

Apply unsupported blending modes in the shader