ui/gtk-egl: apply scale factor when calculating window's dimension

Scale factor needs to be applied when calculating width/height of the
GTK windows.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20231012222643.13996-1-dongwon.kim@intel.com>