C64 multicolor : allow background color to be != 0
|Reported by:||Thomas Bernard||Owned by:||pulkomandy|
current code assumes the C64 background color for Multicolor mode is always 0 (black).
This is very often the case, but not always, this is not even always the most used color in a picture.
I think we should offer a way for the user to select the background color in the GUI. We could also allow him to set the border color.