LOD mesh simplification

[bfops]

#55 needs to happen first, so that we're only representing surfaces with our voxels.

Once that's done, there will be a few considerations to make, like whether we want progressive meshes (storing each mesh along with a list of incremental "simplification" operations) or more quantized clipmaps (in which case, we might want some kind of "mipmesh" system, in which we store each mesh with recursively halved numbers of triangles).

http://procworld.blogspot.com/2011/10/clipmaps.html
http://procworld.blogspot.com/2011/10/playing-dice-with-mesh-simplification.html

[bfops]

we should also add a clipmap view; e.g. a render mode that shows clipmap rings as distinct colors.

[bfops]

fbf764d and c764c4f do this for now.