Make actors outside map borders selectable for their owner

PR #5967 only made planes not lose their selection when they strayed
outside the map border. This PR makes it possible to select them when
they already are outside the map.

Fixes #5651 for real, then.
Could be. I'm not home atm, so I can't check. If I had to choose between those two problems, I'd say that was the lesser evil, though. Not being able to select your aircraft while playing is way more annoying than during a replay.