Tests.ExternalTexture.Msaa: open frame 2 before waiting on startup()

The MSAA tests deadlocked after merging master into rework-thread-model.

The render thread waits on startupDone while the JS thread runs the
AddToContextAsync .then() callback. That callback calls startup() ->
new NativeEngine() / new Scene() / new RenderTargetTexture(), which
eventually hits SubmitCommands. SubmitCommands now synchronously
acquires a FrameCompletionScope, which blocks while m_frameBlocked
is true. The gate only opens on the next StartRenderingCurrentFrame --
but the render thread is stuck on startupDone. Deadlock.

Fix: open frame 2 before waiting on startupDone, and reuse the same
frame for renderFrame(). Any JS work that touches the engine has to run
while a frame is in progress under the new model.

This workaround is temporary. Once #1646 lands, the test should migrate
from AddToContextAsync to the new synchronous CreateForJavaScript, at
which point startup() runs in the same JS task as the texture creation
and the cross-frame dance disappears entirely.

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

Author: bghgary

Apps/UnitTests/Source/Tests.ExternalTexture.Msaa.cpp

@@ -89,12 +89,17 @@ namespace
         addToContextCalled.get_future().wait();
         update.Finish();
         device.FinishRenderingCurrentFrame();
-        startupDone.get_future().get();
 
-        // New frame: drive a single renderFrame() on the JS side.
+        // Open the next frame BEFORE waiting for startup() to complete. The AddToContextAsync .then()
+        // callback runs on the JS thread and calls startup(), which constructs NativeEngine + Scene + RTT,
+        // each step submitting bgfx commands. In the threading model from #1652, SubmitCommands
+        // synchronously acquires a FrameCompletionScope and blocks until a frame is in progress, so a
+        // frame must be open while startup() runs. The same frame is reused for renderFrame().
         device.StartRenderingCurrentFrame();
         update.Start();
 
+        startupDone.get_future().get();
+
         std::promise<void> renderDone;
         loader.Dispatch([&renderDone](Napi::Env env) {
             auto jsPromise = env.Global().Get("renderFrame").As<Napi::Function>().Call({}).As<Napi::Promise>();