Make the Mesh api more robust

[jespertheend]

• There's a bunch of TODO's in there that still need some investigation.
• Private mesh methods should be marked as such #784
• There's a setVertexData but not getVertexData on the Mesh class. Would be nice to have.
• copyAttributeBufferData() (which should be private) will contain some functionality in the future to directly copy buffer data without converting to math types. It would be nice to have an api to allow users to do this as well. When merging multiple meshes into one for instance, it can be very useful to transfer vertex data from one mesh to another.
• copyBufferData should be removed or made private. Users can pass an ArrayBuffer to setVertexData or directly modify the underlying ArrayBuffers instead.
• Add Mesh.clone() #833
• Support for meshes without index buffers #858
• Using Mesh.setVertexCount() to decrease the vertex count throws an error #887
• Mesh.clone() keeps a reference to the old mesh around in attribute buffers. #888
• Don't expose MeshAttributeBuffer #907