Fixed GIF remapping to palette with duplicate entries

[radarhere]

Addresses #6389

remap_palette cannot map multiple palette indexes to the same output index, by its very definition. It accepts a list of palette indexes that it transforms to the position in the list. There cannot be more than one palette index at a given position in the list.

If a palette is supplied when saving a GIF image, im.save("out.gif", palette=...), and that palette has duplicate entries, then the following GifImagePlugin code will try to prepare a list that maps the first of the duplicated entries to multiple locations, leaving the rest of the duplicate entries to be not strictly defined in the behaviour of remap_palette.

Pillow/src/PIL/GifImagePlugin.py

Lines 519 to 526 in ad7be55

	used_palette_colors = []
	for i in range(0, len(source_palette), 3):
	source_color = tuple(source_palette[i : i + 3])
	try:
	index = im.palette.colors[source_color]
	except KeyError:
	index = None
	used_palette_colors.append(index)

This PR fixes that, by leaving the duplicated indexes as they are - setting index to None, the next loop in GifImagePlugin

Pillow/src/PIL/GifImagePlugin.py

Lines 527 to 532 in ad7be55

	for i, index in enumerate(used_palette_colors):
	if index is None:
	for j in range(len(used_palette_colors)):
	if j not in used_palette_colors:
	used_palette_colors[i] = j
	break

will tell remap_palette to map the index to itself, leaving it untouched.

[Yay295]

I think the if could be inside the try, since if there's an exception it will have already been set to None.

[radarhere]

I actually have a different idea of how to simplify the code. In my latest commit, I replace the try block with get().