fixed graphical bugs when using background graphics on menu screens

This bug occurs when using background graphics on menu screens in
combination with a bottom screen tape panel (without using the normal
request door, but using envelope style door requests instead). This
caused problems with black boxes in the bottom right corner on all
sub-screens (like info, setup or scores screens).

This bugfix now also allows non-black backgrounds for bottom screen
tape panels (by defining the main screen to use the whole window,
including the area reserved for the bottom screen tape panel, which
uses the main screen's background graphics during sliding in or out).