Deprecate P;2L and P;4L in frombytes()

[radarhere]

Sequel to #9725

That PR raised a ValueError if P;2L or P;4L data is truncated in frombytes(). This PR deprecates their use as independent unpackers altogether. They can still be used when decoding PCX images.

I've moved the unpackers from Unpack.c to PcxDecode.c. Once the deprecation period is up, the unpackers and RAWMODES entries can be removed.

[akx]

Hmm, these could still be useful decoders for people using Pillow to eg. carve images out of, say, game binaries?

[radarhere]

Could you expand further? What makes you think they are used for anything other than PCX images?

[akx]

Sure. I've been involved in projects to port retro games, where Pillow has been useful to convert custom bespoke file formats to modern ones. That particular example isn't P;2L or P;4L, sure, just a palettized image with RLE encoding, but in general Pillow's decoders being available to "userland" have been useful for this sort of thing.

I don't particularly see a reason why this should be deprecated in frombytes - what's the rationale? It isn't clear from the PR description.

[radarhere]

Because #9725 added rather specific code for the handling of these two rawmodes in a non-PCX context

Pillow/src/PIL/Image.py

Lines 953 to 957 in cfc8a1e

	if decoder_args and decoder_args[0] in {"P;2L", "P;4L"}:
	multiple = 4 if decoder_args[0] == "P;2L" else 8
	if len(data) % multiple:
	msg = "not enough image data"
	raise ValueError(msg)

and if the rawmodes aren't necessary for any other encoder, I'd like to get rid of it.