Terrain Engine Experiment

This is some old experiments in a terrain engine for a globe.  The code
is definitely experimental and not suitable for any particular purpose.

The main idea is to make an engine which does LOD on patches, where
the patch structure is fixed which allows for efficient submission to
hardware (whereas per-triangle LOD algorithms are hard to make efficient
for hardware).

The other thing is that the number of patches is fixed - they're reused
and recycled as needed, but there should be no allocator overhead.
I don't know how useful this is in practice (it needs a proper priority
queue rather than a sorted list to make this significant, I think).