Separate framebuffer sampling params from tex.

This way we don't need to reset params on the texture or worry about
things like that.  We always force on the framebuffer anyway, so this is
simpler.

Also don't enable mipmaps when using a framebuffer, since they don't make
sense currently.
Auto-disable shader blending after 48 per frame.

Should prevent possible massive slowdowns from log reporting.
Merge pull request #6336 from unknownbrackets/texcache

Separate framebuffer sampling params from tex