setScreenBackground() on missionscreens with backgroundSpecial clears/hides map.

[CmdrSvengali]

When working on the unified handling for the background AddOns I've noticed that you can't set the background-images without losing the map on missionscreens which are using backgroundSpecial, even if the image uses an alpha channel. Using setScreenOverlay() instead as workaround works pretty well though, but it's just that - a workaround.

To reproduce (with a BGS image):

1. mission.runScreen({backgroundSpecial:"LONG_RANGE_CHART"})
2. setScreenBackground("bgs_inter_ov.png")