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/palette.py

@@ -3,7 +3,7 @@ from typing import Dict, Type
 
 import colormath.color_objects
 
-from colours import DHGRColours
+from colours import HGRColours
 
 # Type annotation
 RGB = colormath.color_objects.sRGBColor
@@ -24,7 +24,7 @@ class BasePalette:
     ID = Palette.UNKNOWN  # type: Palette
 
     # Palette RGB map
-    RGB = {}  # type: Dict[DHGRColours: RGB]
+    RGB = {}  # type: Dict[HGRColours: RGB]
 
 
 class NTSCPalette(BasePalette):
@@ -32,22 +32,22 @@ class NTSCPalette(BasePalette):
 
     # Palette RGB values taken from BMP2DHGR's default NTSC palette
     RGB = {
-        DHGRColours.BLACK: rgb(0, 0, 0),
-        DHGRColours.MAGENTA: rgb(148, 12, 125),
-        DHGRColours.BROWN: rgb(99, 77, 0),
-        DHGRColours.ORANGE: rgb(249, 86, 29),
-        DHGRColours.DARK_GREEN: rgb(51, 111, 0),
-        DHGRColours.GREY1: rgb(126, 126, 126),
-        DHGRColours.GREEN: rgb(67, 200, 0),
-        DHGRColours.YELLOW: rgb(221, 206, 23),
-        DHGRColours.DARK_BLUE: rgb(32, 54, 212),
-        DHGRColours.VIOLET: rgb(188, 55, 255),
-        DHGRColours.GREY2: rgb(126, 126, 126),
-        DHGRColours.PINK: rgb(255, 129, 236),
-        DHGRColours.MED_BLUE: rgb(7, 168, 225),
-        DHGRColours.LIGHT_BLUE: rgb(158, 172, 255),
-        DHGRColours.AQUA: rgb(93, 248, 133),
-        DHGRColours.WHITE: rgb(255, 255, 255)
+        HGRColours.BLACK: rgb(0, 0, 0),
+        HGRColours.MAGENTA: rgb(148, 12, 125),
+        HGRColours.BROWN: rgb(99, 77, 0),
+        HGRColours.ORANGE: rgb(249, 86, 29),
+        HGRColours.DARK_GREEN: rgb(51, 111, 0),
+        HGRColours.GREY1: rgb(126, 126, 126),
+        HGRColours.GREEN: rgb(67, 200, 0),
+        HGRColours.YELLOW: rgb(221, 206, 23),
+        HGRColours.DARK_BLUE: rgb(32, 54, 212),
+        HGRColours.VIOLET: rgb(188, 55, 255),
+        HGRColours.GREY2: rgb(126, 126, 126),
+        HGRColours.PINK: rgb(255, 129, 236),
+        HGRColours.MED_BLUE: rgb(7, 168, 225),
+        HGRColours.LIGHT_BLUE: rgb(158, 172, 255),
+        HGRColours.AQUA: rgb(93, 248, 133),
+        HGRColours.WHITE: rgb(255, 255, 255)
     }
 
 
@@ -56,22 +56,22 @@ class IIGSPalette(BasePalette):
 
     # Palette RGB values taken from BMP2DHGR's KEGS32 palette
     RGB = {
-        DHGRColours.BLACK: rgb(0, 0, 0),
-        DHGRColours.MAGENTA: rgb(221, 0, 51),
-        DHGRColours.BROWN: rgb(136, 85, 34),
-        DHGRColours.ORANGE: rgb(255, 102, 0),
-        DHGRColours.DARK_GREEN: rgb(0, 119, 0),
-        DHGRColours.GREY1: rgb(85, 85, 85),
-        DHGRColours.GREEN: rgb(0, 221, 0),
-        DHGRColours.YELLOW: rgb(255, 255, 0),
-        DHGRColours.DARK_BLUE: rgb(0, 0, 153),
-        DHGRColours.VIOLET: rgb(221, 0, 221),
-        DHGRColours.GREY2: rgb(170, 170, 170),
-        DHGRColours.PINK: rgb(255, 153, 136),
-        DHGRColours.MED_BLUE: rgb(34, 34, 255),
-        DHGRColours.LIGHT_BLUE: rgb(102, 170, 255),
-        DHGRColours.AQUA: rgb(0, 255, 153),
-        DHGRColours.WHITE: rgb(255, 255, 255)
+        HGRColours.BLACK: rgb(0, 0, 0),
+        HGRColours.MAGENTA: rgb(221, 0, 51),
+        HGRColours.BROWN: rgb(136, 85, 34),
+        HGRColours.ORANGE: rgb(255, 102, 0),
+        HGRColours.DARK_GREEN: rgb(0, 119, 0),
+        HGRColours.GREY1: rgb(85, 85, 85),
+        HGRColours.GREEN: rgb(0, 221, 0),
+        HGRColours.YELLOW: rgb(255, 255, 0),
+        HGRColours.DARK_BLUE: rgb(0, 0, 153),
+        HGRColours.VIOLET: rgb(221, 0, 221),
+        HGRColours.GREY2: rgb(170, 170, 170),
+        HGRColours.PINK: rgb(255, 153, 136),
+        HGRColours.MED_BLUE: rgb(34, 34, 255),
+        HGRColours.LIGHT_BLUE: rgb(102, 170, 255),
+        HGRColours.AQUA: rgb(0, 255, 153),
+        HGRColours.WHITE: rgb(255, 255, 255)
     }