Add support for ImageOps invert, posterize and solarize of P images

[RavSinghChandan]

Changes proposed in this pull request:

• ImageOps.invert(), posterize() and solarize() raised NotImplementedError: mode P support coming soon for a P image, from the FIXME in _lut(): "apply to lookup table, not image data".
• These three change each pixel on its own, so for a P image the lookup table can be applied to the palette colors instead. The result is the same as applying it to the RGB image, and alpha in an RGBA palette is left unchanged.
• autocontrast() and equalize() build their table from the histogram, which for a P image counts palette indexes rather than colors, so they still raise as before.
• Tests check each operation against its RGB result, that palette alpha is kept, and that autocontrast still refuses P.

[radarhere]

equalize() build their table from the histogram, which for a P image counts palette indexes rather than colors, so they still raise as before.

_lut() doesn't actually need to be concerned about this, as equalize() converts P to RGB by itself.

Pillow/src/PIL/ImageOps.py

Lines 495 to 496 in b01a706

	if image.mode == "P":
	image = image.convert("RGB")

[RavSinghChandan]

You're right, thanks, my description was wrong about equalize(). The guard only matters for autocontrast(), which builds its table from image.histogram(), and for a P image that counts palette indexes. Your version looks good to me.