Fixed some issues with units getting stuck on the blocking grid

Hopefully it won't happen again, but I'm still skeptical. Important
changes:

* Don't allow skipping corners (required pathfinding algorithm to be
  switched to A*)
* Fixed some coordinate offset problems when determining line-of-sight
* Fixed current unit position to be taken into account
Clicking will not set target position for the minion master

Though he's still refusing to actually walk there.