Allocate a vertex scratchpad per thread only once per frame
Allocate a vertex scratchpad per thread only once per frame

This moves the allocation of the vertex scratchpad out of Planet::subdivide and into
Planet's WorkerThread implementation.  This means that there is an allocation per
thread per frame, instead of an allocation per icosahedron face per frame.  So this
doesn't make a great deal of a difference, and the comment suggesting that the code
is relatively hot is no longer quite as relevant.

StrayLightning/aeolius,StrayLightning/aeolius,StrayLightning/aeolius