Proof of concept DHGR encoding/playback

- 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.

transcoder/opcodes.py

@@ -76,13 +76,18 @@ class Ack(Opcode):
     """Instructs player to perform TCP stream + buffer management."""
     COMMAND = OpcodeCommand.ACK
 
+    def __init__(self, aux_active: bool):
+        self.aux_active = aux_active
+
     def emit_data(self) -> Iterator[int]:
-        # Dummy bytes to pad out TCP frame
-        yield 0xff
+        # Flip $C054 or $C055 soft-switches to steer subsequent writes to
+        # MAIN/AUX screen memory
+        yield 0x54 if self.aux_active else 0x55
+        # Dummy byte to pad out TCP frame
         yield 0xff
 
     def __data_eq__(self, other):
-        return True
+        return self.aux_active == other.aux_active
 
 
 class BaseTick(Opcode):