added test urdf files for minitaur with all fixed joints, or fixed knees.
added some stiffness/damping to minitaur legs (testing)
tiny_obj_loader, don't crash on invalid texture coordinates
btMultiBodyConstraintSolver: sweep back and forward to reduce asymmetry
add minitaur quadruped playback of minitaur log files (real robot and simulated create the same log files)
add improved minitaur.urdf file, see https://youtu.be/lv7lybtOzeo for a preview.