Added GimpPaletteFile frombytes() to allow for unlimited parsing

[radarhere]

Resolves #6639

#6640 has concerns about Pillow's limits when reading a Gimp palette file.

Pillow/src/PIL/GimpPaletteFile.py

Lines 33 to 34 in e66ebb6

	for _ in range(256):
	s = fp.readline()

1. Pillow reads at most 256 lines after the first header line. This means that Pillow can't read more than 256 colours, which apparently might be the case in Gimp palette files and the spec doesn't say otherwise. I'm concerned about the idea of creating a "palette" that is larger than what our ImagePalette can handle.
2. The fact that only 256 lines are read after the first header line also means that any additional lines that are present ('Name:' header, 'Columns:' header, comments) actually subtract from the 256 colour limit.

Pillow/src/PIL/GimpPaletteFile.py

Lines 41 to 43 in e66ebb6

	if len(s) > 100:
	msg = "bad palette file"
	raise SyntaxError(msg)

3. A colour's name can't be long enough to make the line more than 100 characters.

My general concern is that the spirit of #515 is against reading an unlimited amount of data from a file.

The test files that we have do include two additional headers and a comment.

Pillow/Tests/images/custom_gimp_palette.gpl

Lines 1 to 5 in e66ebb6

	GIMP Palette
	Name: custompalette
	Columns: 4
	#
	0 0 0 Index 3

So I think extending the number of lines read by three is reasonable. That is my first commit, partly addressing point 2.

Beyond that, I think there are three options.
A. Do nothing. Palettes with more than 256 colours and long colour names are an edge case, and this is a lesser used format.
B. #6640's solution is to add class variables to allow configuration - max_colors, _max_line_size and _max_file_size. I think that makes for a bit of a complex API.
C. My idea is to add a frombytes() method that accepts bytes. That way, if the user doesn't like our limits, they can take on the responsibility of reading everything from a file, and we can just parse the result.

[aclark4life]

Yeah I like frombytes too and I agree with you that adding class variables is not great.