FR: Make AI asteroid intercept code and autobrackets account for gravity

[Durandal-Thoth]

While @Baezon told me the lead indicator accounts for gravity, other engine features do not, in particular the AI targeting asteroids which have no chance of colliding with escort targets as well as chasing down ones which pass them. The engine also fails to properly bracket asteroids which are on a collision course.

https://www.youtube.com/watch?v=kzvhYHSHXPo