Bad asteroids need fixing

Issue #2199 highlighted the fact that asteroid planets were completely ruined.

This attempts to fix them by clamping the range of values they can actually produce. It still produces details but they're much smoother.

Problems I've seen with this so far are that cracks between patches are much more visible with this method. I imagine it's due to the scaling and limiting of the range and being multiplied several times etc.

At least now they can be landed on even if they look like crap!