GPU rework

[dnotq]

The GPU needs to be reworked to fix these problems:

1. Change from a logic-based control to a microcoded control as per the original TMS9900.
2. Change the ALU to be a real ALU instead of a big fat mux.
3. Remove the expectation that memory access is always a single cycle, and remove the fixed wait-points.

Requires coordination with the external memory access to mux/round-robin access to VRAM between the GPU and the host-interface FSM.