Implement in-shader blending on gles2 / gl2

I'm sure it'll be slow, though... but, better than being black, I guess?

Keeping the texelFetch path since I expect it to be faster/more accurate / save uniforms.

-[Unknown]
I think so, but then there's floating point error.  When I changed it to LINEAR the outlines in Dragon Ball Z looked better at some render resolutions.  So either I'm doing something wrong or it matters.

I checked and gl_fragCoord is centered (0.5, 0.5 is the bottom left most pixel.)

-[Unknown]