Update post_processing example to not render UI with first pass camera (#6469)

# Objective

Make sure the post processing example won't render UI twice.

## Solution

Disable UI on the first pass camera with `UiCameraConfig`

examples/shader/post_processing.rs

@@ -95,19 +95,24 @@ fn setup(
     });
 
     // Main camera, first to render
-    commands.spawn(Camera3dBundle {
-        camera_3d: Camera3d {
-            clear_color: ClearColorConfig::Custom(Color::WHITE),
+    commands.spawn((
+        Camera3dBundle {
+            camera_3d: Camera3d {
+                clear_color: ClearColorConfig::Custom(Color::WHITE),
+                ..default()
+            },
+            camera: Camera {
+                target: RenderTarget::Image(image_handle.clone()),
+                ..default()
+            },
+            transform: Transform::from_translation(Vec3::new(0.0, 0.0, 15.0))
+                .looking_at(Vec3::default(), Vec3::Y),
             ..default()
         },
-        camera: Camera {
-            target: RenderTarget::Image(image_handle.clone()),
-            ..default()
-        },
-        transform: Transform::from_translation(Vec3::new(0.0, 0.0, 15.0))
-            .looking_at(Vec3::default(), Vec3::Y),
-        ..default()
-    });
+        // Disable UI rendering for the first pass camera. This prevents double rendering of UI at
+        // the cost of rendering the UI without any post processing effects.
+        UiCameraConfig { show_ui: false },
+    ));
 
     // This specifies the layer used for the post processing camera, which will be attached to the post processing camera and 2d quad.
     let post_processing_pass_layer = RenderLayers::layer((RenderLayers::TOTAL_LAYERS - 1) as u8);