[Ogre] Optional Ogre tread damage rule

[David Morse]

More thoughts:

1) The Ogre Miniatures equivalent of this rule is to just pro-rate the
tread points into movement.  So if you've got a Mark V with 45 out of
60 treads, your V moves at 45/60th of 6" ... 4.5".  Lose another tread
point, movement drops to 4.4".  It is quite elegant if you have a
decimal ruler.

2) (back in hex-based Ogre) Rolling the die at the beginning of the
movement phase is more humane than rolling when you try to use the MP
and fail.