C64 multicolor : allow background color to be != 0

[Thomas Bernard]

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.

function Check_block_constraints()
​https://gitlab.com/GrafX2/grafX2/blob/master/src/buttons_effects.c#L239
function Pixel_in_screen_c64multi_with_opt_preview()
​https://gitlab.com/GrafX2/grafX2/blob/master/src/graph.c#L3456

[Thomas Bernard]

We could use the "transparent color" set in the "layer" menu as the C64 background.