Optimize vertex buffers

[kkaefer]

We could optimize vertex buffers by making sure we don't duplicate any vertices because we're using indexed draws (with drawElements) everywhere anyway. This reduces the memory, bandwidth and allows the hardware to the transformed vertex cache so it only has to transform a vertex once.

To better use that cache, we should sort the vertex and index buffers so that we avoid large jumps: ideally, indices are as linear as possible. To achieve this, we can reorder the vertex cache.

[kkaefer]

(Note that this may not yield a lot of speed wins because of the way we generate our index buffers; they're mostly ascending already)

[kkaefer]

We are doing this now.