headless_renderer: copy render target after RenderQueue has been submitted

[rectalogic]

Objective

Currently the headless_renderer example copies the render target in the RenderGraph schedule. The copy happens at the beginning of the schedule before the current frame has been submitted to the RenderQueue. So it is always copying the render results from the previous frame.

Solution

Schedule the copy after RenderGraphSystems::Submit

Testing

To test, temporarily modify headless_renderer to set single_image: false and pre_roll_frames to 0 and to stamp each render with the frame number - using the patch below.

Before this PR, the rendered frames were:

• 000.png transparent
• 001.png black (due to RenderStartup causes an additional frame of latency in rendering (aka pink screen part 2) #20756)
• 002.png "frame 1"
• 003.png "frame 2"
• ...

With this PR, the rendered frames are:

• 000.png black (due to RenderStartup causes an additional frame of latency in rendering (aka pink screen part 2) #20756)
• 001.png "frame 1"
• 002.png "frame 2"
• ...

diff --git a/examples/app/headless_renderer.rs b/examples/app/headless_renderer.rs
index 82107edd7..fb977d12e 100644
--- a/examples/app/headless_renderer.rs
+++ b/examples/app/headless_renderer.rs
@@ -71,7 +71,7 @@ fn main() {
     let config = AppConfig {
         width: 1920,
         height: 1080,
-        single_image: true,
+        single_image: false,
     };
 
     // setup frame capture
@@ -165,10 +165,12 @@ fn setup(
         // 2. Few black box images
         // 3. Fully rendered scene images
         // Exact number depends on device speed, device load and scene size
-        40,
+        0,
         "main_scene".into(),
     );
 
+    commands.spawn(Text::new("initial"));
+
     // Scene example for non black box picture
     // circular base
     commands.spawn((
@@ -193,6 +195,7 @@ fn setup(
 
     commands.spawn((
         Camera3d::default(),
+        IsDefaultUiCamera,
         render_target,
         Tonemapping::None,
         Transform::from_xyz(-2.5, 4.5, 9.0).looking_at(Vec3::ZERO, Vec3::Y),
@@ -457,7 +460,12 @@ fn update(
     mut scene_controller: ResMut<SceneController>,
     mut app_exit_writer: MessageWriter<AppExit>,
     mut file_number: Local<u32>,
+    mut text: Single<&mut Text>,
+    mut frame_number: Local<u32>,
 ) {
+    text.0 = format!("frame {}", *frame_number);
+    *frame_number += 1;
+
     if let SceneState::Render(n) = scene_controller.state {
         if n < 1 {
             // We don't want to block the main world on this,