Chunk LoD

[Ralith]

As of #63, we're GPU bound on weak hardware. Using simplified geometry for distant chunks could help alleviate this. A very simple approach would be to run marching cubes on entire chunks, such that each chunk is a single cell, containing only a few triangles. Challenges include:

• Preventing geometric gaps between LoD levels
• Addressing the discontinuity between boxy and smooth terrain (if we retain boxy voxels)
• Texturing the topologically very distinct geometry (if we retain boxy voxels)

[Ralith]

transvoxel presents an elegant solution for LoD boundary stitching with marching cubes.

[Ralith]

This should not be addressed before #41, which will reduce the number of triangles rendered dramatically and shift the bottleneck.

[Ralith]

@patowen's benchmarking currently indicates that the rendering time for voxels is utterly insignificant compared to CPU costs, so we should not invest in chunk LoD for the foreseeable future.