Now that units flee it's necessary to fix the unit chasing: this commit
reintroduces some code from 298115f4c5 that disappeared with the
committing of the new pathfinder. Refs #1537.
Also includes some style improvements to the UnitMotion code.
This was SVN commit r17013.
Add short-range vertex-based pathfinder.
Integrate new pathfinder into unit motion code.
Change obstruction system to get rid of circles, and differentiate
structures from units.
Make PositionChanged messages synchronous.
Try to prevent some accidental float->int conversions.
This was SVN commit r7484.