`CardPainter::set_background_image_path()` immediately repaints the card
back and inverted card back bitmaps if they exist, so users just need
to `update()` that part of the screen. This is handled automatically by
`CardGame`, but other users will have to do this manually.
For now, the only feature of this is that it sets the background colour
from the `Games::Cards::BackgroundColor` config value. Later, other
card game configuration and shared behaviour can go here, to save
duplicating it in each game.
