Core: Don't pass through a reference to the window handle

Now that MainNoGUI is properly architected and GLX doesn't need to
sometimes craft its own windows sometimes which we have to thread back
into MainNoGUI, we don't need to thread the window handle that GLX
creates at all.

This removes the reference to pass back here, and the g_pWindowHandle
always be the same as the window returned by Host_GetRenderHandle().

A future cleanup could remove g_pWindowHandle entirely.

Source/Core/VideoBackends/D3D/main.cpp

@@ -141,7 +141,7 @@ void VideoBackend::ShowConfig(void *hParent)
 	Host_ShowVideoConfig(hParent, GetDisplayName(), "gfx_dx11");
 }
 
-bool VideoBackend::Initialize(void *&window_handle)
+bool VideoBackend::Initialize(void *window_handle)
 {
 	if (window_handle == nullptr)
 		return false;