OpenCS: Render cell markers

This is gugus's work (PR  #187).  All I did was merge in the master branch and made it compile with gcc on ubuntu.

Re-implemented using overlays.
I don't see the point in converting the ManualObject to a mesh if you're only ever creating one instance (entity) of the mesh. You can just attach the ManualObject to a scene node instead.

This is also inefficient, it creates a new vertex buffer for each rendered cell line. You could achieve the different positions by moving scene nodes around, instead of a creating new vertex buffer.