Change source mesh for instances

Hi!

This is a test I did in the babylon playground. It showcases changing the source mesh of instances meshes. It seems to work but I would very much want feedback on if there is a cleaner way to do this.

https://www.babylonjs-playground.com/#LS60WB#7

I think instancing examples are a bit too few on the playground so maybe this one could be good to include once it is done as it shows how to hook up instance buffers to a ShaderMaterial as well as changing the sourceMesh.

Cheers,
Anders.