UnitMotion - make ComputeTargetPosition return a position for point goals

This makes it possible to merge the TryGoingStraightTo family of function.
This means it is now called even for point goals and decreases the likelihood of stuck units (as we take over from the long-range pathfinder which brings units to navcell centers, which may not be in the goal).

Differential Revision: https://code.wildfiregames.com/D1982