Core: Render a blank UI frame before booting

This is related to https://bugs.dolphin-emu.org/issues/10958 which
uses Qt to clear out the window so the game list isn't displayed
while the core is booting. Instead, we use the video backend to
render a black screen, which means Qt doesn't have to flip between
paint engines.

Source/Core/Core/Core.cpp

@@ -470,6 +470,11 @@ static void EmuThread(std::unique_ptr<BootParameters> boot, WindowSystemInfo wsi
   }
   Common::ScopeGuard video_guard{[] { g_video_backend->Shutdown(); }};
 
+  // Render a single frame without anything on it to clear the screen.
+  // This avoids the game list being displayed while the core is finishing initializing.
+  g_renderer->BeginUIFrame();
+  g_renderer->EndUIFrame();
+
   if (cpu_info.HTT)
     SConfig::GetInstance().bDSPThread = cpu_info.num_cores > 4;
   else