Start implementing SAPLayer creation and insertion.

src_testbed/nphysics_backend.rs

@@ -228,7 +228,11 @@ fn nphysics_collider_from_rapier_collider(
         }
     };
 
-    let density = if is_dynamic { collider.density() } else { 0.0 };
+    let density = if is_dynamic {
+        collider.density().unwrap_or(0.0)
+    } else {
+        0.0
+    };
 
     Some(
         ColliderDesc::new(shape)

src_testbed/physx_backend.rs

@@ -237,7 +237,7 @@ impl PhysxWorld {
                 let densities: Vec<_> = rb
                     .colliders()
                     .iter()
-                    .map(|h| colliders[*h].density())
+                    .map(|h| colliders[*h].density().unwrap_or(0.0))
                     .collect();
 
                 unsafe {