Bug#894519: xpra 2.4 depends on python-gtkglext1

[Jeremy Bicha]

Control: tags -1 -buster +buster-ignore +bullseye
Control: block -1 by 885497

Ok, let's keep python-gtkglext1 around for Buster but I do intend to
remove this and anything else still using pygtk for Bullseye.

Thanks,
Jeremy Bicha