Re-introduce contact force visualization

Author: rohanpsingh

mujoco_viewer/mujoco_viewer.py

@@ -112,6 +112,7 @@ def _key_callback(self, window, key, scancode, action, mods):
         elif key == glfw.KEY_C:
             self._contacts = not self._contacts
             self.vopt.flags[mujoco.mjtVisFlag.mjVIS_CONTACTPOINT] = self._contacts
+            self.vopt.flags[mujoco.mjtVisFlag.mjVIS_CONTACTFORCE] = self._contacts
         elif key == glfw.KEY_J:
             self._joints = not self._joints
             self.vopt.flags[mujoco.mjtVisFlag.mjVIS_JOINT] = self._joints