Enable OES_element_index_uint

[ianmackenzie]

The OES_element_index_uint extension is listed as a "universally supported extension", and enabling it should allow elm-explorations/webgl to support indexed meshes with a basically unlimited number of vertices (theoretically billions instead of the current 65536 vertex limit).

This will also require updating the JS code to use Uint32Array instead of Uint16Array.