Allow libtiff to write COLORMAP tag

[radarhere]

Taking a part of #2288

At the moment, COLORMAP is the TiffImagePlugin blocklist, tags that are blocked when saving.

Pillow/src/PIL/TiffImagePlugin.py

Lines 1520 to 1524 in a78de17

	# The other tags expect arrays with a certain length (fixed or depending on
	# BITSPERSAMPLE, etc), passing arrays with a different length will result in
	# segfaults. Block these tags until we add extra validation.
	blocklist = [
	COLORMAP,

This PR adds that validation, throwing an error if it does not have length 768.

The tag is treated as special when writing because libtiff handles it specially - https://gitlab.com/libtiff/libtiff/-/blob/master/libtiff/tif_dir.c#L374-379