added a seesaw to test linear and angular constraints
taking wingmesh dependence out of physics

physics will only need points for gjk, and tris for volume integration, so shapes just keep these now.  todo: remove from constructors in the api.
wingmesh convenience functions Adj(),Next(),Prev() are ref (not pointer) based now.   note use refs, they are still brittle depend on id being member of wingmesh arrays