Optimize framebuffer binding and blits

In one area of Tales of Phantasia X, there is a hot town where there are a ton of intra-buffer blits done in a row each frame.  This town only ran at 95% for me at best on desktop.

Avoiding the temporary fbo in an intra-buffer blit helped a lot, bringing it up to 150%.  With the other optimizations (avoiding unbinding and fbo caching), it's now up to 190%.

This is of course an extreme case.  Another example, Trails in the Sky, seemed to benefit only about 1% from these changes in certain cases, and probably less most of the time.

Requires hrydgard/native#234.

-[Unknown]