Add damping support + demos.

examples2d/all_examples2.rs

@@ -12,6 +12,7 @@ use std::cmp::Ordering;
 
 mod add_remove2;
 mod collision_groups2;
+mod damping2;
 mod debug_box_ball2;
 mod heightfield2;
 mod joints2;
@@ -55,6 +56,7 @@ pub fn main() {
     let mut builders: Vec<(_, fn(&mut Testbed))> = vec![
         ("Add remove", add_remove2::init_world),
         ("Collision groups", collision_groups2::init_world),
+        ("Damping", damping2::init_world),
         ("Heightfield", heightfield2::init_world),
         ("Joints", joints2::init_world),
         ("Platform", platform2::init_world),