Temporarily cap multitexture painting at 4.

My card can actually handle 16, but apparently doing all that makes something
blow out a max size on something.

It'd all fit early on, but I'd capped myself for OTHER performance reasons,
but apparently in the current state of things, it's a no-go.

I have a sneaking suspicion it's that I need to shave off some more in
usableMaxUniformVectors to compensate for how big v_TexMults has grown, but
for the moment this unbreaks QuadShader for the weekend.