Allow Visual component to link directly to a Mesh

I need this as I start working towards procedural content, where another
process will be generating mesh data that needs to make it into the
renderer, but I don't want / need to name the mesh.
Queue up rendering of any meshes directly linked to a Visual

Otherwise this was falling back to mesh name lookup and just showing the
default.
Fast fail when loading / rendering an invalid Mesh
Work snapshot: Implementing marching cubes for volume rendering