Hack in some support for reading ply files

* Uses the nice minimal rply library
* Horrible hacked resource management which must be fixed real soon -
  reads the ply file per frame!
Debug plotting of edges of mesh
Various cleanups for mesh display
Fix ply file vertex index element name
Correct positioning and shading for meshes

* Make sure positioning of meshes is correct with respect to world
  coordinates.  Use float32 positions for storage efficiency, coupled
  with an offset for large origin displacements.

* Fix mesh face shader to provide a simple lambertian lighting with
  faceted normals for ease in debugging meshes.
Allow meshes to be loaded via gui

* Clean up to do allocation and manage mesh lifetimes correctly
* Connect up File->Open to mesh loading functions
Point size in meters + render points as spheres
Fix projected z scaling in sphere imposter rendering
Feedback while shuffling buckets
Load and draw line segments (using .ply format)
Fix offsets when drawing points
Fixes for windows compile
Fix view snapping to work with meshes and lines