* add support for drawing more than 3d plot lines (more than
  drawVList()).  the interface should be generalized where the default for a
  given display type is built in (similar to default overlap callback in librt)
  but can be overridden by a calling user.  useful by interfaces like rtgl where
  there is a custom application display driver.

* explore the possibility of using knowledge of dm pixel size paired with an
  RTree to reduce the number of drawn vlist line segments.  For a large model
  zoomed out,  many line segments may be inside the same voxel "pixel box" -
  we should be able to characterize that and produce some sort of reduced
  vlist tree automatically to ease the drawing load for OpenGL.  May or may
  not be doable in a way that improves performance, but leveraging the RTree
  work done for the meshing code it may be relatively simple to test...
