putpalette() rawmode may be modes that can be unpacked to RGB

[radarhere]

Resolves #1280 by better documenting the range of raw modes that im.putpalette() can accept. It does not only accept RGB or RGBA, but also any raw mode that can be unpacked to RGB.

This means that the limit for the length of data is not just 768 or 1024. Since the limit is the number of channels multiplied by 256, for "R", it is 256.

>>> from PIL import Image
>>> im = Image.new("L", (1, 1))
>>> im.putpalette((1,)*256, "R")
>>> im.putpalette((1,)*257, "R")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/Image.py", line 1776, in putpalette
    self.load()  # install new palette
  File "PIL/Image.py", line 838, in load
    palette_length = self.im.putpalette(mode, arr)
ValueError: invalid palette size