CTBL and SHAM: dynamic color tables

Amiga hardware makes it possible to use a different color map for each scan
line, greatly increasing the number of colors in an image. The semistandard
CTBL and SHAM chunks contain per-line color maps. The more common CTBL chunk,
was defined by NewTek.

offset	length	name	description
------------------------------------------------------------------------------
0	4	ID	"CTBL"
4	4	size	Chunk data size, 32 times the number of rows
8	32*N	cmap	Color map entries
------------------------------------------------------------------------------

Each color map contains 16 entries. Each entry is two bytes. Within each entry,
the low fours bit of the first byte are the red value. The high four bits of
the second byte are the green value and the low four bits are the blue value.

The SHAM chunk, used by other programs, is the same as CTBL except that the
ID is "SHAM", there is a two-byte version field that is currently always zero
before the color map, and so the length is two bytes greater than a
corresponding CTBL chunk.

A non-HAM image with a CTBL chunk is known as dymanic hires. A HAM image with
a CTBL chunk is known as Dynamic HAM or DHAM, one with a SHAM chunk is known
as Sliced HAM or SHAM.