to optimize this, and seems reasonably fast, but not fast enough to
enable it for all transparent meshes. Hopefully the artists can improve
performance by merging meshes together, and we can strike a happy
balance between sorting by mesh and sorting by triangles.
git-svn-id: svn+ssh://src.earth.threerings.net/nenya/trunk@29 ed5b42cb-e716-0410-a449-f6a68f950b19