[BUG] EmbreeScene can only add one primitive of one kind at the moment.

[orgarten]

See blazert/embree/scene.h:

1. add_spheres, add_planes, add_cylinders only adds the first object currently
2. return geomID of this object
3. all other objects in vectors will be ignores

This has to change in some way that we can return the geom_id in the same way as in for blazertscene, otherwise this is not really transparent.