Roam and Wms optimizations

- Store WmsCacheNode's in RoamTriangle and RoamPoint and shortcut the
  fetch operation when they do not change.

- Added RoamView that stores GL_{MODELVIEW,PROJECTION,VIEWPORT_MATRIX}s
  between calls to gluProject.

- Cache heights (via height_func calls) and screen coordinates in
  RoamPoint to avoid excess gluProject calls.