Small optimizations in the emulated GPU

This does a couple things:
 * Adds `gstate_c.framebufChanged`, to skip the `SetRenderFrameBuffer()` logic.  Games usually do a bunch of prims to the same buffer, so this saves time.
 * Does the same thing to time_update() as is done in syscalls.
 * Adds a little more disasm code.

No real huge boost in performance but shaves off a few percent in some games, though usually these games are GPU-limited anyway.

-[Unknown]