Terrain performance improvement

Don't get too excited.  This is small.

For the Everest [terrain example](http://localhost:8080/Apps/Sandcastle/index.html?src=Terrain.html), this brings `createPotentiallyVisibleSet()` from 1.04% to 0.42% by avoiding redundant frustum and horizon culling.

Later, hierarchical commands may help even more, but I don't have them thought through yet.  Oh, and I'll think about making `DrawCommand` more cache-friendly.