RGBa to RGB color conversion mapping

[bigcat88]

Fixes #6706

Added mapping for RGBa to RGB conversion

[radarhere]

RGBa is "true color with premultiplied alpha". In main, converting from RGBA to RGBa makes the RGB values smaller

>>> from PIL import Image
>>> im = Image.new("RGBA", (1, 1))
>>> im.putpixel((0, 0), (150, 175, 200, 100))
>>> im.convert("RGBa").load()[0, 0]
(59, 69, 78, 100)

so converting to RGB should make the RGB values bigger again, yes? Your test shows the RGB values getting smaller.

I've created #6708 instead.