Pad COLORMAP to 768 items when saving TIFF

[radarhere]

Resolves #6230

#6060 contained a commit entitled "Allow getpalette() to return less than 256 colors"

That has that meant that there are less entries in COLORMAP.

Pillow/src/PIL/TiffImagePlugin.py

Lines 1677 to 1679 in c6637bc

	if im.mode in ["P", "PA"]:
	lut = im.im.getpalette("RGB", "RGB;L")
	ifd[COLORMAP] = tuple(v * 256 for v in lut)

Which then triggers the following error in the issue.

Pillow/src/encode.c

Lines 791 to 792 in c6637bc

	PyErr_SetString(
	PyExc_ValueError, "Requiring 768 items for Colormap");

This PR pads the COLORMAP entries until it reaches 768.