Improve transparency handling when saving GIF images

[radarhere]

Resolves #6162
Resolves #6260

When saving a GIF, _normalize_mode may set info["transparency"] for a frame.

Pillow/src/PIL/GifImagePlugin.py

Line 482 in a575ec3

im.info["transparency"] = im.palette.colors[rgba]

When the code comes back to _write_multiple_frames, the transparency is used, if the user hasn't overridden it from encoderinfo... but it is only copied from the first frame.

Pillow/src/PIL/GifImagePlugin.py

Lines 573 to 576 in a575ec3

	im_frame = _normalize_mode(im_frame.copy())
	if frame_count == 0:
	for k, v in im_frame.info.items():
	im.encoderinfo.setdefault(k, v)

This PR allows each frame to have a different transparency from _normalize_mode.

[raygard]

Where you now have:

if "transparency" in im_frame.info:
	encoderinfo.setdefault("transparency", im_frame.info["transparency"])
im_frame = _normalize_palette(im_frame, palette, encoderinfo)

Does it hurt anything to put the im_frame = ... in front?

im_frame = _normalize_palette(im_frame, palette, encoderinfo)
if "transparency" in im_frame.info:
	encoderinfo.setdefault("transparency", im_frame.info["transparency"])

I ask because it will help my issue #6260 with transparency if they can be swapped around like this.

[radarhere]

_remap_palette, as the name suggests, reorders the palette. However, it doesn't update the transparency index, meaning that they can become out of sync. This is what happens in #6260. I've added a commit here to fix that.

Also using the swap mentioned in the previous comment, that means that when _normalize_palette calls _remap_palette, the updated transparency can then be used for the frame.

[raygard]

Looks good! Your approach is much cleaner than mine was. I hope this can get into the next release.