Allow fewer PNG palette entries than the bit depth maximum when saving

[radarhere]

Resolves #2202

Currently when saving a PNG in P mode, Pillow sets the number of bytes in the palette at two to the power of the bit depth, times 3 for the colors.

Pillow/src/PIL/PngImagePlugin.py

Lines 1272 to 1274 in 8064e04

	if im.mode == "P":
	palette_byte_number = (2 ** bits) * 3
	palette_bytes = im.im.getpalette("RGB")[:palette_byte_number]

As pointed out in the issue, https://www.w3.org/TR/PNG/#11PLTE states that

It is permissible to have fewer entries than the bit depth would allow.

So this PR changes palette_byte_number to the number of colors times 3 instead.

The length of the tRNS chunk is also changed, as https://www.w3.org/TR/PNG/#11tRNS

The tRNS chunk shall not contain more alpha values than there are palette entries