mmp_geodesics: opengl performance improvements
mmp_geodesics: keep opengl usable after sanity_check failed
gl::SurfaceDrawable: implement INVISIBLE_FACE_MODE and SOLID_FACE_MODE
mmp_geodesics: Some cosmetics
mmp_geodesics: rename Surface to TriSurface
mmp_geodesics: make distance_function an independent class
mmp_geodesics: call GeodesicsInspector from main()
mmp_geodesics: don't insert non-frontier boundary events.
Retrieve distances at vertices from adjacent frontier events.

To label the vertices with boundary events define MMP__USE_LABELING_EVENTS
mmp_geodesics: use std::make_heap/push_heap/pop_heap to realize the PriorityQueue
mmp_geodesics: fix iterator corruption in Geodesics::update_event_points leading to errors if labeling events are used / if MMP__USE_LABELING_EVENTS is defined.
cosmetics