Use VAO to save on vertex specifying commands when drawing

The new behavior becomes active if WebGL on the client system supports
OES_vertex_array_object. The VAO reference is stored on the geometry
chunk. If the interleaved buffer is dirtied and the VAO references it, it
is reconstructed based on the regular buffers. The VAO is also reset if
the chunk is rebuilt. VAOs are not used if morph geometry is present.

OES_vertex_array_object is a widely supported WebGL extension, around
75% of clients have it according to webglstats. In large scenes on
CPU-constrained systems that support the extension this patch can
increase FPS as much as 15%.
Merge pull request #317 from NVIDIA/use-VAO

Use VAO to save on vertex specifying commands when drawing