viewport_node example: Remove main world image initialization

[SpecificProtagonist]

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.

[chompaa]

Unfortunately, after this PR the viewport_node example looks like this for me:

Note the distorted image.

[JMS55]

I didn't test the example admittedly.

[grind086]

The main world access is used by update_viewport_render_target_size to update the texture when the viewport node's size changes.

bevy/crates/bevy_ui/src/widget/viewport.rs

Lines 154 to 176 in 4051465

	pub fn update_viewport_render_target_size(
	viewport_query: Query<
	(&ViewportNode, &ComputedNode),
	Or<(Changed<ComputedNode>, Changed<ViewportNode>)>,
	>,
	camera_query: Query<&Camera>,
	mut images: ResMut<Assets<Image>>,
	) {
	for (viewport, computed_node) in &viewport_query {
	let camera = camera_query.get(viewport.camera).unwrap();
	let size = computed_node.size();
	let Some(image_handle) = camera.target.as_image() else {
	continue;
	};
	let size = Extent3d {
	width: u32::max(1, size.x as u32),
	height: u32::max(1, size.y as u32),
	..default()
	};
	images.get_mut(image_handle).unwrap().resize(size);
	}
	}

[SpecificProtagonist]

Thanks, fixed! <3 Also removed the query for the window size, as the result gets overwritten anyways.

[chompaa]

Works as expected now, thanks!