More robust normals

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Vertex normals are generated from the heightfield right now. This doesn't work for "manual" changes by the user, like voxel removal.

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We can keep track of the vertex normals on the fly when we generate polys, by adding the face normal to the normal at each index. There'll be artifacts at block boundaries.

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Fixing those artifacts could be the start of the skirt-generating code to fix mesh seams at LOD switches (#103).

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7f00b71 creates normals during mesh generation. It looks really terrible though. I think it's got a bug.

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Can debug this by rendering just the normals without lighting or color. Relates to #52.

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Normals shouldn't be inferred on the fly, because it takes away the ability to be specific about how they should look when creating the voxel tree (which will be important if we want to import meshes).

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In particular, the difference between the images below isn't algorithmic quality; it depends on preference for how sharp or rounded the terrain should look. Procworld points out in some early posts that support for both is key: we want to be able to create nice round terrain, as well as sharp features, e.g. from mesh imports, or for buildings. To do that, we need to be able to specify our normals in the voxel tree like before, instead of inferring them during polygon generation.

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This is being handled specifically for specific purposes, e.g. #137