Groundcover should ignore non-geometry Drawables

An example of affected mesh: v_sponge2.nif

This mesh leads to floating fish in OpenMW. That's because the fish is implemented as a particle system inside spounge mesh, while OSG supports instancing only for Geometry objects. MGE ignores this particle system when this mesh is used for groundcover.

A solution is to ignore Drawables which are not derived from Geometry.