Add a video Mode enum.

transcoder/video.py

@@ -1,5 +1,6 @@
 """Encode a sequence of images as an optimized stream of screen changes."""
 
+import enum
 import heapq
 import os
 import queue
@@ -18,6 +19,11 @@ import opcodes
 import screen
 
 
+class Mode(enum.Enum):
+    HGR = 0
+    DHGR = 1
+
+
 class Video:
     """Apple II screen memory map encoding a bitmapped frame."""