Codechange: Off-by-one in colour gradient initialisation.

Remap sprites start with a count byte followed by 256 entries, but
SetupColoursAndInitialWindow did not take account of this extra byte and
therefore started at palette index 0xC5 instead of 0xC6. This caused the
first colour of each gradient to be incorrect and all shades were actually
1 step lower in the gradient than indicated.

Codechange: Remove direct access to _colour_gradient.

Access is now through GetColourGradient, which ensures parameters are in range.

Add: 32bpp-to-8bpp palette index lookup.

Lookups are calculated on demand and caches in a 256KB in-memory table.