M2(DX9): dual-mode DX9 mock-game render backend

Add render_dx09.cpp (selectable as `dx9ex` / `dx9`): renders the animated
pattern with Clear + ColorFill (D3D9's built-in rect fill -- no shaders) and
presents through a real D3D9 / D3D9Ex device. Two modes so the two D3D9 capture
paths each have a matching game. Smoke-verified: both present frames and exit
cleanly. The capture hook + frame-decode test land next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/mock_game/render_backend.cpp

@@ -17,6 +17,14 @@ std::unique_ptr<RenderBackend> RenderBackend::create(const std::string& name)
 	{
 		return create_dx10_backend();
 	}
+	if (name == "dx9ex")
+	{
+		return create_dx9_backend(/*ex=*/true);
+	}
+	if (name == "dx9")
+	{
+		return create_dx9_backend(/*ex=*/false);
+	}
 	return nullptr;
 }