Support for meshes without index buffers

[jespertheend]

Right now the Mesh constructor always creates an empty index buffer, and there is no way to to create a mesh without an index buffer.
I think it should be null by default, and only when setting index data should it be created.
Similarly, setIndexData should probably take null as a value, causing the index buffer to be removed.

Also the WebGPU renderer doesn't really handle this case well.