Clamp terrain m_maxHeightInMeters to prevent insane spike-balls

We've had problems with overly spiky terrains for ages. Issue #1680.

The problem seems to be partly caused by some bodies being given an excessively large `m_maxHeightInMeters`. This clamps it to avoid crazy spike-balls.

(We'll have to revisit this in the future, to allow more extreme bodies without placing surface starports on them).

Before:
![screenshot-20130527-140055](https://f.cloud.github.com/assets/115749/568129/8176b994-c6cd-11e2-8d78-6d21aff3d397.png)

After:
![screenshot-20130527-135858](https://f.cloud.github.com/assets/115749/568118/56f36ee2-c6cd-11e2-89eb-412b7ea91471.png)