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'll note that my math shouldn't be wrong, because of the way the absdiff outlines are done, if I was off by any real factor, it would not draw them properly at all (in fact it was wrong at first, I made a typo in the uniform name under glGetUniformLocation, arg, so it was always 0.)

So I can only assume it's float error...

-[Unknown]