Buildfix.
Note the location of a memset variant.
Premultiply for doubled alpha blending if possible.

This makes the effect more accurate where the values were being clamped
before.  Some obscure methods of blending may be slower.
Avoid recalculating replaceBlend so many times.
Avoid premultiplying easy fixed colors.
Dirty the fixed color when premultiplying.
Avoid using fbo copies for the common 2a + 1-2a.

We won't even get it right that often since the copy doesn't deal with
overlap properly.
Improve a possible obscure blend mode.

Seems like this would be likely to get overlap problems.
Optimize fixed color blending that isn't blending.

Gods Eater Burst uses this, for example.
More correctly blend when hitting the frame cap.

If we hit too many blits per frame, fall back correctly in the shader.
Add a hack for Silent Hill games.

Either way has different problems, but it sounds like this will work
better for Silent Hill.  Probably still rendering wrong, unfortunately.