Improve splash damage falloff calculation to account for obstruction size.

Splash falloff was calculated using centre-to-point distance, where nearest-edge to nearest-edge ought to have been used.
Use DistanceToPoint to correct for that.

Make sure the damage multiplier cannot go negative.
Remove GetUnitSize in favour of GetSize.

Reviewed By: bb

Differential Revision: https://code.wildfiregames.com/D2963