Add collision groups demos.

examples3d/all_examples3.rs

@@ -11,6 +11,7 @@ use rapier_testbed3d::Testbed;
 use std::cmp::Ordering;
 
 mod add_remove3;
+mod collision_groups3;
 mod compound3;
 mod debug_boxes3;
 mod debug_cylinder3;
@@ -65,6 +66,7 @@ pub fn main() {
     let mut builders: Vec<(_, fn(&mut Testbed))> = vec![
         ("Add remove", add_remove3::init_world),
         ("Primitives", primitives3::init_world),
+        ("Collision groups", collision_groups3::init_world),
         ("Compound", compound3::init_world),
         ("Domino", domino3::init_world),
         ("Heightfield", heightfield3::init_world),