Copy Python palette to new image in quantize()

[radarhere]

Resolves #5695

quantize() accepts palette as an argument, but only applies the palette to the C layer.

Pillow/src/PIL/Image.py

Lines 1123 to 1133 in d50052a

	if palette:
	# use palette from reference image
	palette.load()
	if palette.mode != "P":
	raise ValueError("bad mode for palette image")
	if self.mode != "RGB" and self.mode != "L":
	raise ValueError(
	"only RGB or L mode images can be quantized to a palette"
	)
	im = self.im.convert("P", dither, palette.im)
	return self._new(im)

This PR copies the palette into the Python layer as well.

[paulds8]

Looking at the change, I don't quite understand how adding the 'GIF' format to the save worked though. My expectation was that the issue would be somewhere in save() but if this works I'm happy.

[radarhere]

When writing the PNG PLTE chunk, the palette data is being retrieved from the C layer (.im), but limited to the number of colors

Pillow/src/PIL/PngImagePlugin.py

Lines 1283 to 1288 in d50052a

	if im.mode == "P":
	palette_byte_number = colors * 3
	palette_bytes = im.im.getpalette("RGB")[:palette_byte_number]
	while len(palette_bytes) < palette_byte_number:
	palette_bytes += b"\0"
	chunk(fp, b"PLTE", palette_bytes)

colors is set here (unless there is no palette or you're passing a "bits" argument, but those are not the case)

Pillow/src/PIL/PngImagePlugin.py

Line 1206 in d50052a

colors = max(min(len(im.palette.getdata()[1]) // 3, 256), 1)

This is determining the number of colors from the ImagePalette object in the Python layer

Pillow/src/PIL/ImagePalette.py

Lines 80 to 89 in d50052a

	def getdata(self):
	"""
	Get palette contents in format suitable for the low-level
	``im.putpalette`` primitive.
	.. warning:: This method is experimental.
	"""
	if self.rawmode:
	return self.rawmode, self.palette
	return self.mode, self.tobytes()

So in this PR, I am making sure that ImagePalette is also correct, meaning that the number of colors is correct when saving to PNG.