Implement GPU block copies between buffers and from buffer to RAM.

(first one not very well tested, second one improves Burnout Legends sun a lot).

Implements parts of #3515 and #618.

Also hopes to be an alternative to #6030.

Intended for Buffered Rendering, not the Read Framebuffer modes.

I hope that with this we can minimize the use of the Read Framebuffer modes (they will still be required in Dangan Ronpa).