Add damping support + demos.

src_testbed/testbed.rs

@@ -405,8 +405,19 @@ impl Testbed {
     }
 
     pub fn set_world(&mut self, bodies: RigidBodySet, colliders: ColliderSet, joints: JointSet) {
+        self.set_world_with_gravity(bodies, colliders, joints, Vector::y() * -9.81)
+    }
+
+    pub fn set_world_with_gravity(
+        &mut self,
+        bodies: RigidBodySet,
+        colliders: ColliderSet,
+        joints: JointSet,
+        gravity: Vector<f32>,
+    ) {
         println!("Num bodies: {}", bodies.len());
         println!("Num joints: {}", joints.len());
+        self.gravity = gravity;
         self.physics.bodies = bodies;
         self.physics.colliders = colliders;
         self.physics.joints = joints;