Backtrack on special hexcode, support 24bit mode
Backtrack on special hexcode, support 24bit mode

There's no auto-detect, but it's still trivial to just set the mode
with `Paint.mode = 0xffffff` or `2**24-1` or however you like.
It doesn't actually check for the exact value, if it's not one
of the other supported modes then it's 24-bit.

janlelis/paint