Common: Add a render_window field to WindowSystemInfo

We need this because we need to pass the layer to MoltenVK, not
the view handle. But the input subsystem still needs the window.

Source/Core/DolphinNoGUI/PlatformFBDev.cpp

@@ -91,6 +91,7 @@ WindowSystemInfo PlatformFBDev::GetWindowSystemInfo() const
   WindowSystemInfo wsi;
   wsi.type = WindowSystemType::FBDev;
   wsi.display_connection = nullptr;  // EGL_DEFAULT_DISPLAY
+  wsi.render_window = nullptr;
   wsi.render_surface = nullptr;
   return wsi;
 }