[webgl-point] Use OES_element_index_uint when available

Currently we use the `UNSIGNED_SHORT` type with `drawElements`. This means we cannot draw more than `65536 / 4 = 16384` points. This PR suggests using the `OES_element_index_uint` extension when available. With that extension we can draw up to up to 1 billion points, which gives us some air. The extension is available in most WebGL-capable browsers as reported on http://webglstats.com/.