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.
We may have added this to avoid problems in observer mode. https://github.com/OpenRA/OpenRA/commit/707c86fbbbb3f0e30a31045ed59fa5d968c3460a