Use OpenGL 3.2 extension ARB_draw_elements_base_vertex, which allows us
to add offset in index buffer indices automatically. This should reduce
the number of draw calls significantly.
We may have some errors on the Mac with VBO setting off.
OSX OpenGL extensions don't play well with vertex arrays.
Will test that more later.
We might also use a full element buffer here, like we do when hiding
some quads, but this approach keeps the memory savings intended
originally.
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