- Every time we process an ACK opcode, toggle page 1/page 2 soft
switches to steer subsequent writes between MAIN and AUX memory
- while I'm here, squeeze out some unnecessary operations from the
buffer management
On the player side, this is implemented by maintaining two screen
memory maps, and alternating between opcode streams for each of them.
This is using entirely the wrong colour model for errors, but
surprisingly it already works pretty well in practise (and the frame
rate is acceptable on test videos)
DHGR/HGR could be made runtime selectable by adding a header byte that
determines whether to set the DHGR soft switches before initiating
the decode loop.
While I'm in here, fix op_terminate to clear keyboard strobe before
waiting.
