Approximate better with two fixed blend colors

The idea here is to at least allow one to be a constant, and to allow more variance in the comparisons.  This is basically an improved version of #1614.

I hope it's not worse for performance (using a Vec3), but the common case seems to be the 0xFFFFFF/0x000000 by far, which should be fine.

This makes the pause screen in Exit look much better, fixes the menu fade in Popolocrois, and at least doesn't seem to make anything look awful in Lunar.

-[Unknown]