Checkpoint WIP for easier comparison to dhgr branch:

- naive version of NTSC artifacting, it uses a sliding 4-bit window to
  assign a nominal (D)HGR colour to each dot position.  A more
  sophisticated/correct implementation would model the YIQ signal
  directly.

- Switch DHGRBitmap implementation to use a 34-bit representation of
  the 4-byte tuple, comprised of a 3-bit header and footer, plus
  4*7=28-bit body.  The headers/footers account for the influence on
  neighbouring tuples from the 4-bit NTSC window.

- With this model each screen byte influences 13 pixels, so we need to
  precompute 2^26 edit distances for all possible (source, target)
  13-bit sequences.

- Checkpointing not-yet-working HGR implementation.

- Add new unit tests but not yet all passing due to refactoring

transcoder/video_test.py

@@ -21,6 +21,7 @@ class TestVideo(unittest.TestCase):
         frame.page_offset[0, 1] = 0b1010101
 
         target_pixelmap = screen.DHGRBitmap(
+            palette = palette.Palette.NTSC,
             main_memory=v.memory_map,
             aux_memory=frame
         )
@@ -28,10 +29,10 @@ class TestVideo(unittest.TestCase):
             0b0000000101010100000001111111,
             target_pixelmap.packed[0, 0])
 
-        diff = v._diff_weights(v.pixelmap, target_pixelmap, is_aux=True)
-
         pal = palette.NTSCPalette
 
+        diff = target_pixelmap.diff_weights(v.pixelmap, is_aux=True)
+
         # Expect byte 0 to map to 0b00000000 01111111
         expect0 = target_pixelmap.edit_distances(pal.ID)[0][0b0000000001111111]
 
@@ -63,7 +64,7 @@ class TestVideo(unittest.TestCase):
             target_pixelmap.packed[0, 0]
         )
 
-        diff = v._diff_weights(v.pixelmap, target_pixelmap, is_aux=True)
+        diff = target_pixelmap.diff_weights(v.pixelmap, is_aux=True)
 
         # Expect byte 0 to map to 0b01111111 01101101
         expect0 = target_pixelmap.edit_distances(pal.ID)[0][0b0111111101101101]