viewport_node example: Remove main world image initialization (#19098)

# Objective

The new viewport example allocates a texture in main memory, even though
it's only needed on the GPU. Also fix an unnecessary warning when a
viewport's texture doesn't exist CPU-side.

## Testing

Run the `viewport_node` example.

crates/bevy_ui/src/widget/viewport.rs

@@ -171,6 +171,11 @@ pub fn update_viewport_render_target_size(
             height: u32::max(1, size.y as u32),
             ..default()
         };
-        images.get_mut(image_handle).unwrap().resize(size);
+        let image = images.get_mut(image_handle).unwrap();
+        if image.data.is_some() {
+            image.resize(size);
+        } else {
+            image.texture_descriptor.size = size;
+        }
     }
 }

examples/ui/viewport_node.rs

@@ -2,17 +2,14 @@
 //! pick entities visible in the widget's view.
 
 use bevy::{
-    image::{TextureFormatPixelInfo, Volume},
+    asset::RenderAssetUsages,
     picking::pointer::PointerInteraction,
     prelude::*,
     render::{
         camera::RenderTarget,
-        render_resource::{
-            Extent3d, TextureDescriptor, TextureDimension, TextureFormat, TextureUsages,
-        },
+        render_resource::{TextureDimension, TextureFormat, TextureUsages},
     },
     ui::widget::ViewportNode,
-    window::PrimaryWindow,
 };
 
 fn main() {
@@ -29,7 +26,6 @@ struct Shape;
 
 fn test(
     mut commands: Commands,
-    window: Query<&Window, With<PrimaryWindow>>,
     mut images: ResMut<Assets<Image>>,
     mut meshes: ResMut<Assets<Mesh>>,
     mut materials: ResMut<Assets<StandardMaterial>>,
@@ -37,31 +33,16 @@ fn test(
     // Spawn a UI camera
     commands.spawn(Camera3d::default());
 
-    // Set up an texture for the 3D camera to render to
-    let window = window.single().unwrap();
-    let window_size = window.physical_size();
-    let size = Extent3d {
-        width: window_size.x,
-        height: window_size.y,
-        ..default()
-    };
-    let format = TextureFormat::Bgra8UnormSrgb;
-    let image = Image {
-        data: Some(vec![0; size.volume() * format.pixel_size()]),
-        texture_descriptor: TextureDescriptor {
-            label: None,
-            size,
-            dimension: TextureDimension::D2,
-            format,
-            mip_level_count: 1,
-            sample_count: 1,
-            usage: TextureUsages::TEXTURE_BINDING
-                | TextureUsages::COPY_DST
-                | TextureUsages::RENDER_ATTACHMENT,
-            view_formats: &[],
-        },
-        ..default()
-    };
+    // Set up an texture for the 3D camera to render to.
+    // The size of the texture will be based on the viewport's ui size.
+    let mut image = Image::new_uninit(
+        default(),
+        TextureDimension::D2,
+        TextureFormat::Bgra8UnormSrgb,
+        RenderAssetUsages::all(),
+    );
+    image.texture_descriptor.usage =
+        TextureUsages::TEXTURE_BINDING | TextureUsages::COPY_DST | TextureUsages::RENDER_ATTACHMENT;
     let image_handle = images.add(image);
 
     // Spawn the 3D camera