AI no longer calls support power on things it can't target

This makes the AI take into consideration whether or not it can target a unit when considering how attractive a target it is for its support power - if it can't be targetted then it's not considered.

I tested this change by creating a large number of stealth tanks and having them sat clumped together as bait - when cloaked the AI didn't air strike them and instead chose another target. When uncloaked (I made them force fire on a bit of terrain) it saw them and air striked them (as they were by far the most obvious target).

I think this will make it a bit more fun to use stealth tanks against the AI - it decimated my stealth tanks the other day and I was a little bit displeased (╯°□°)╯︵ ┻━┻