Fix mesh seams

[bfops]

There are gaps between different LODs. http://procworld.blogspot.ca/2013/07/emancipation-from-skirt.html

[bfops]

Maybe no block should generate polys that cross its edges, and instead we should run seam fixing code between any two adjacent blocks, regardless of LOD.

[bfops]

It would be useful to enforce the property that a voxel is Surface if any of its higher-LOD breakdowns are Surfaces. Right now, a voxel is (usually) Empty if all of its corners are on the same side of the surface, but it's possible that the surface enters and exits the voxel between corners in a way that's picked up by higher LOD versions. The problem is that when there are adjacent voxels of different LODs, they disagree on where corners exist, so they can disagree on where to draw polygons.

[bfops]

Given ^that property, mesh extraction should be rewritten to connect any adjacent voxels regardless of LOD. Then we can support voxels that stay big because they just don't have any more detail. That lets us bump up our LOD a lot, because we won't be paying the cost in places where the detail doesn't exist.

[bfops]

The server should support an extra request kinda like RequestSeamBetween(BlockPosition, LOD, BlockPosition, LOD)

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Wait until after #143

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I think this will be easier once #170 is done, since the edges between chunks have different behavior from the edges within chunks.

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I've started this in the fix-mesh-seams branch.