Fix another race condition with multithreading.

It's possible that after it's marked completed, the CPU will grab it to
enqueue.  This actually happened to mean with decent reproducibility.
Fix GL errors / flicker in framebuf->mem.

Happens in Tales of Phantasia X, reported by Kingcom.
Use SSE some in framebuffers to memory.

Since people are using it (even though it seems dangerous), might as well
make it faster.  6% better in Tales of Phantasia X.
Merge pull request #6013 from unknownbrackets/gpu-minor

Fix another race, fix bug in CPU framebuf download, speed up