#3 Implement constraints in the physics code

[Gordon Tisher]

The rigid-body physics system (currently only single bodies in gsgl::physics::rigid_body) should be extended to implement constraints.

See http://www.cs.cmu.edu/~baraff/papers/sig96.pdf for a suggested approach.

Also, Game Programming Gems 4, section 3.4, has a clear description of how to set up the Lagrange multipliers for various kinds of constraints.