* implement support for plotting point clouds to DM ps, plot, tk

* make sure lighting is enabled by default

* provide libdm mechanism for drawing spline curves (to help visualize
  flight paths, show object trajectories, etc.).

* release dm X get_color() memory

* decouple libdm from librt -- just one file in libdm uses librt,
  including a whole nasty primitive switch statement.  need to push
  that back over into librt.

* break cyclic dependency with libged, enable screengrab without libged being aware of libdm
