Improved ImagePalette

[radarhere]

This PR has several changes

• the ImagePalette colors dictionary is currently empty when the object is created, rather than being filled with palette entries. This fixes that
• it changes the palette to be empty by default. This means that new colors can be appended more easily, rather than finding that all 256 entries are already occupied
• if a new color is added when 256 entries are taken, search through the image. If there is a palette index that isn't actually used, use that for the new color
• helps ImageOps.expand distorts image and converts png to grayscale #5375 by determining the border palette index on the new image only after the palette has been attached

Edit: Thanks to later commits, resolves #2803

[radarhere]

I've pushed another commit to not use ImagePalette.raw in convert. Since this code -

Pillow/src/PIL/Image.py

Lines 1012 to 1019 in 52856bc

	new.palette = ImagePalette.raw("RGB", new.im.getpalette("RGB"))
	if delete_trns:
	# This could possibly happen if we requantize to fewer colors.
	# The transparency would be totally off in that case.
	del new.info["transparency"]
	if trns is not None:
	try:
	new.info["transparency"] = new.palette.getcolor(trns)

is currently always going to raise a ValueError.

Pillow/src/PIL/ImagePalette.py

Lines 142 to 144 in 52856bc

	def raw(rawmode, data):
	palette = ImagePalette()
	palette.rawmode = rawmode

Pillow/src/PIL/ImagePalette.py

Lines 90 to 96 in 52856bc

	def getcolor(self, color):
	"""Given an rgb tuple, allocate palette entry.
	.. warning:: This method is experimental.
	"""
	if self.rawmode:
	raise ValueError("palette contains raw palette data")

This fixes the UserWarning in #2803

[radarhere]

Using https://pypi.org/project/apng/ to split Tests/images/apng/mode_palette_alpha.png into frames, this matches the transparent third frame

[radarhere]

Expanding the test from #5551, bringing it closer to #5375

[radarhere]

I've now pushed a commit, 'Fixed reloading palette'. It allows this code to work.

from PIL import Image
im = Image.open("Tests/images/hopper.gif")
im.load()
im.palette.dirty = 1
im.save("out.png")

In master at the moment, it produces this broken output.

And with the addition of that change, this PR now resolves #2803