Do less memcpy when copying the framebuf to mem

I'm still convinced there's ways to do this smarter, but I wondered why it was so slow.  For me, this improves e.g. 3rd Birthday by like 40% when this is on.  It's still like half the speed or something compared to when off, but better than 1/3.

Probably could kill the malloc/free every time too, and it could probably be changed to happen at the end of draw/list sync (and at every render switch) instead of copy display to output.  Technically it doesn't copy the last rendered FBO to memory afaict, but I haven't checked carefully, since it copies only when the render FB changes.

Also, I know the framebuffer height is estimated wrong in cases, in which this probably writes too much memory.

-[Unknown]