Examples: Fix shadows in physics instancing demos. (#30988)

* Examples: Fix shadows in physics instancing demos.

* Clean up.

examples/physics_ammo_instancing.html

@@ -60,14 +60,24 @@
 				dirLight.shadow.camera.zoom = 2;
 				scene.add( dirLight );
 
-				const floor = new THREE.Mesh(
+				const shadowPlane = new THREE.Mesh(
+					new THREE.PlaneGeometry( 10, 10 ),
+					new THREE.ShadowMaterial( {
+						color: 0x444444
+					} ),
+				);
+				shadowPlane.rotation.x = - Math.PI / 2;
+				shadowPlane.receiveShadow = true;
+				scene.add( shadowPlane );
+
+				const floorCollider = new THREE.Mesh(
 					new THREE.BoxGeometry( 10, 5, 10 ),
-					new THREE.ShadowMaterial( { color: 0x444444 } )
+					new THREE.MeshBasicMaterial( { color: 0x666666 } )
 				);
-				floor.position.y = - 2.5;
-				floor.receiveShadow = true;
-				floor.userData.physics = { mass: 0 };
-				scene.add( floor );
+				floorCollider.position.y = - 2.5;
+				floorCollider.userData.physics = { mass: 0 };
+				floorCollider.visible = false;
+				scene.add( floorCollider );
 
 				//
 

examples/physics_jolt_instancing.html

@@ -58,14 +58,24 @@
 				dirLight.shadow.camera.zoom = 2;
 				scene.add( dirLight );
 
-				const floor = new THREE.Mesh(
+				const shadowPlane = new THREE.Mesh(
+					new THREE.PlaneGeometry( 10, 10 ),
+					new THREE.ShadowMaterial( {
+						color: 0x444444
+					} ),
+				);
+				shadowPlane.rotation.x = - Math.PI / 2;
+				shadowPlane.receiveShadow = true;
+				scene.add( shadowPlane );
+
+				const floorCollider = new THREE.Mesh(
 					new THREE.BoxGeometry( 10, 5, 10 ),
-					new THREE.ShadowMaterial( { color: 0x444444 } )
+					new THREE.MeshBasicMaterial( { color: 0x666666 } )
 				);
-				floor.position.y = - 2.5;
-				floor.receiveShadow = true;
-				floor.userData.physics = { mass: 0 };
-				scene.add( floor );
+				floorCollider.position.y = - 2.5;
+				floorCollider.userData.physics = { mass: 0 };
+				floorCollider.visible = false;
+				scene.add( floorCollider );
 
 				//
 

examples/physics_rapier_instancing.html

@@ -58,14 +58,24 @@
 				dirLight.shadow.camera.zoom = 2;
 				scene.add( dirLight );
 
-				const floor = new THREE.Mesh(
+				const shadowPlane = new THREE.Mesh(
+					new THREE.PlaneGeometry( 10, 10 ),
+					new THREE.ShadowMaterial( {
+						color: 0x444444
+					} ),
+				);
+				shadowPlane.rotation.x = - Math.PI / 2;
+				shadowPlane.receiveShadow = true;
+				scene.add( shadowPlane );
+
+				const floorCollider = new THREE.Mesh(
 					new THREE.BoxGeometry( 10, 5, 10 ),
-					new THREE.ShadowMaterial( { color: 0x444444 } )
+					new THREE.MeshBasicMaterial( { color: 0x666666 } )
 				);
-				floor.position.y = - 2.5;
-				floor.receiveShadow = true;
-				floor.userData.physics = { mass: 0 };
-				scene.add( floor );
+				floorCollider.position.y = - 2.5;
+				floorCollider.userData.physics = { mass: 0 };
+				floorCollider.visible = false;
+				scene.add( floorCollider );
 
 				//