Move the infinite fall debug example to the 3D examples.

All our other debug examples were there.

examples3d/debug_infinite_fall3.rs

@@ -0,0 +1,32 @@
+use rapier3d::dynamics::{JointSet, RigidBodyBuilder, RigidBodySet};
+use rapier3d::geometry::{ColliderBuilder, ColliderSet};
+use rapier_testbed3d::Testbed;
+
+pub fn init_world(testbed: &mut Testbed) {
+    /*
+     * World
+     */
+    let mut bodies = RigidBodySet::new();
+    let mut colliders = ColliderSet::new();
+    let joints = JointSet::new();
+
+    let rad = 1.0;
+    // Build the dynamic box rigid body.
+    let rigid_body = RigidBodyBuilder::new_dynamic()
+        .translation(0.0, 3.0 * rad, 0.0)
+        .can_sleep(false)
+        .build();
+    let handle = bodies.insert(rigid_body);
+    let collider = ColliderBuilder::ball(rad).build();
+    colliders.insert(collider, handle, &mut bodies);
+
+    /*
+     * Set up the testbed.
+     */
+    testbed.set_world(bodies, colliders, joints);
+}
+
+fn main() {
+    let testbed = Testbed::from_builders(0, vec![("Boxes", init_world)]);
+    testbed.run()
+}