Reinstate 2-player mode

[joeldipops]

Currently the RSP renderer is only written to handle one player (that is, one input buffer at a time, no support for different scaling, we're relying on the hope that a frame will be rendered before the next one is ready to start. As a result of finally getting this to work, I just hid the options to add a second player. The performance was too atrocious to bear thinking about anyway.

However, we should be able to rewrite the RSP code to handle two players. My plan is to call it once at the end of each logic step, and the input will include addresses for both buffers.

We will also need to implement a signal that doesn't interrupt the RSP until it's finished a frame. Probably will skip the next frame, rather than queue it and let them back up.