correctly set up background color in mobile example (#12384)

# Objective

- Since #11165, the button in the mobile example doesn't visually react
to touches

## Solution

- Correctly set up the background color

examples/mobile/src/lib.rs

@@ -104,7 +104,8 @@ fn setup_scene(
 
     // Test ui
     commands
-        .spawn(ButtonBundle {
+        .spawn((
+            ButtonBundle {
                 style: Style {
                     justify_content: JustifyContent::Center,
                     align_items: AlignItems::Center,
@@ -114,8 +115,11 @@ fn setup_scene(
                     bottom: Val::Px(50.0),
                     ..default()
                 },
+                image: UiImage::default().with_color(Color::NONE),
                 ..default()
-        })
+            },
+            BackgroundColor(Color::WHITE),
+        ))
         .with_children(|b| {
             b.spawn(
                 TextBundle::from_section(