ui/gtk-egl: egl context needs to be unbound in the end of gd_egl_switch

A thread often fails to bind an egl context to itself after guest VM is
rebooted because the context is still owned by another thread. It is not
very clear what condition makes this happen but this can be prevented
by unbinding the context from the thread in the end of gd_egl_switch.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Message-Id: <20220928215805.4661-1-dongwon.kim@intel.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>