[meshing] Type of the indexbuffer

[rcabanier]

From @thetuvix:

required Uint16Array indices;
Depending on the density of mesh requested, a mesh volume on HoloLens can sometimes have enough indices to require a 32-bit index buffer.

[rcabanier]

WebGL does not allow for a 32 bit index buffer by default.
Is it easy to split the mesh into smaller pieces on the UA side?

I see that if the "OES_element_index_uint" extension is available, you can pass a Uint32Array.
Maybe we can have an option where an author can ask for more efficient meshes. I don't think it should be done by default because it might break content if we return 32bit values to code that expects 16bit