IIGS palettes.
Move the palette diff_matrix generation into make_data_tables.py since
that is the only place it is used.
Demand-load the edit distance matrices when transcoding.
(3-pixel) sequences that may be modified when storing bytes to the
DHGR display.
This relies on producing an efficient linear representation of the
DHGR framebuffer in terms of a packed 28-bit representation of (Aux,
Main, Aux, Main) screen bytes.
