Apps: fix dispatch ordering merge artifact between BabylonJS#1652 and BabylonJS#1646

In BabylonJS#1674's combination of rework-thread-model (BabylonJS#1652) and the
ExternalTexture sync API (BabylonJS#1646), three apps swapped the original
async AddToContextAsync flow for synchronous CreateForJavaScript but
kept the two-frame skeleton from the async version. The result is
that the loader.Dispatch which runs the JS startup() callback now
gets queued in frame 1 even though its observable wait
(startup.get_future().wait()) is in frame 2 — the dispatch can land
in either frame depending on JS-thread scheduling, breaking the
"each Start/Finish pair wraps a phase of work" pattern. On
rework-thread-model the JS thread will block on the closed gate
between frames 1 and 2 anyway, so it functionally works, but the
ordering is wrong.

Move the dispatch into the second frame so the texture creation,
startup() call, and the wait that observes them all run inside the
same frame:

  frame 1: load scripts            (no dispatch)
  frame 2: dispatch[startup] -> wait
  frame 3+: per-asset / render-loop

Three apps had this issue; UnitTests' ExternalTexture tests already
do it correctly. On bare rework-thread-model these apps still use
AddToContextAsync and don't have the bug.

Files:
- Apps/HeadlessScreenshotApp/Win32/App.cpp
- Apps/PrecompiledShaderTest/Source/App.cpp
- Apps/StyleTransferApp/Win32/App.cpp

Local build verification (Win32 RelWithDebInfo):
- HeadlessScreenshotApp: builds cleanly
- StyleTransferApp: builds cleanly
- PrecompiledShaderTest: not configured in this Build/Win32; change
  is structurally identical to HeadlessScreenshotApp which compiled.

[Created by Copilot on behalf of @bghgary]

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bghgary

Apps/HeadlessScreenshotApp/Win32/App.cpp

@@ -127,6 +127,15 @@ int main()
     // Create a render target texture for the output.
     winrt::com_ptr<ID3D11Texture2D> outputTexture = CreateD3DRenderTargetTexture(d3dDevice.get());
 
+    // Close the script-load frame.
+    deviceUpdate.Finish();
+    device.FinishRenderingCurrentFrame();
+
+    // Open a new frame for `startup` so the JS-side resource creation and
+    // startup() call run in the same frame as the wait that observes them.
+    device.StartRenderingCurrentFrame();
+    deviceUpdate.Start();
+
     std::promise<void> startup{};
 
     // Create an external texture for the render target texture and pass it to
@@ -142,17 +151,10 @@ int main()
         startup.set_value();
     });
 
-    deviceUpdate.Finish();
-    device.FinishRenderingCurrentFrame();
-
-    // Reopen the gate so JS can continue running (startup may issue bgfx commands).
-    device.StartRenderingCurrentFrame();
-    deviceUpdate.Start();
-
     // Wait for `startup` to finish.
     startup.get_future().wait();
 
-    // Close the frame opened above.
+    // Close the startup frame.
     deviceUpdate.Finish();
     device.FinishRenderingCurrentFrame();
 

Apps/PrecompiledShaderTest/Source/App.cpp

@@ -135,6 +135,15 @@ int RunApp(
     Babylon::ScriptLoader loader{runtime};
     loader.LoadScript("app:///index.js");
 
+    // Close the script-load frame.
+    deviceUpdate.Finish();
+    device.FinishRenderingCurrentFrame();
+
+    // Open a new frame for `startup` so the JS-side resource creation and
+    // startup() call run in the same frame as the wait that observes them.
+    device.StartRenderingCurrentFrame();
+    deviceUpdate.Start();
+
     std::promise<void> startup{};
 
     // Create an external texture for the render target texture and pass it to
@@ -150,17 +159,10 @@ int RunApp(
         startup.set_value();
     });
 
-    deviceUpdate.Finish();
-    device.FinishRenderingCurrentFrame();
-
-    // Reopen the gate so JS can continue running (startup may issue bgfx commands).
-    device.StartRenderingCurrentFrame();
-    deviceUpdate.Start();
-
     // Wait for `startup` to finish.
     startup.get_future().wait();
 
-    // Close the frame opened above.
+    // Close the startup frame.
     deviceUpdate.Finish();
     device.FinishRenderingCurrentFrame();
 

Apps/StyleTransferApp/Win32/App.cpp

@@ -334,6 +334,15 @@ int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
     loader.LoadScript("app:///Scripts/babylonjs.loaders.js");
     loader.LoadScript("app:///Scripts/index.js");
 
+    // Close the script-load frame.
+    g_update->Finish();
+    g_device->FinishRenderingCurrentFrame();
+
+    // Open a new frame for `startup` so the JS-side resource creation and
+    // startup() call run in the same frame as the wait that observes them.
+    g_device->StartRenderingCurrentFrame();
+    g_update->Start();
+
     std::promise<void> startup{};
 
     // Create an external texture for the render target texture and pass it to
@@ -349,17 +358,10 @@ int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
         startup.set_value();
     });
 
-    g_update->Finish();
-    g_device->FinishRenderingCurrentFrame();
-
-    // Reopen the gate so JS can continue running (startup may issue bgfx commands).
-    g_device->StartRenderingCurrentFrame();
-    g_update->Start();
-
     // Wait for `startup` to finish.
     startup.get_future().wait();
 
-    // Close the frame opened above.
+    // Close the startup frame.
     g_update->Finish();
     g_device->FinishRenderingCurrentFrame();