280: Add NEG_DIR_MAG to lights.txt spec, lights.txt

[tngreene]

There is an optimization for one very very special case.

Take an airplane_nav_left_size light, aim it completely down the X- axis. It should be simply the DIR_MAG but, negative. Instead there is a 180 static rotation to aim the light since the exporter doesn't know that it really could just be a negative DIR_MAG and it would work.

• [ ] Update lights spec with NEG_DIR_MAG psuedo parameter
• [ ] Update lights.txt's airplane_nav_left_size entry to use NEG_DIR_MAG
• [ ] Add unit tests and etc for the parser
• [ ] parser code
• [ ] Anywhere you see DIR_MAG make a copy where it gets multiplied by -1

The math should simply work out at that point (I think?)

This a very minor case, I think. It'd be worth asking artists how often they aim their lights like this. In my experiance most artists aren't so careful, if there is even a real life plane that has EXACTLY orthogonal nav lights like that.