Remove rounded corners on emulation render window

On Windows 11, when playing windowed in a separate window/widget from the main emulator window, we don't want the window to have rounded corners, as it prevents the corner pixels from being visible

Source/Core/DolphinQt/RenderWidget.h

@@ -34,6 +34,7 @@ signals:
 
 private:
   void HandleCursorTimer();
+  void OnHandleChanged(void* handle);
   void OnHideCursorChanged();
   void OnNeverHideCursorChanged();
   void OnLockCursorChanged();