x11: xpra compatibility degredation

[haraldschilly]

just for the record, I tried to update xpra to 4.1.3-r1-1 and that's not working well. that's really slow. same for one of the newest versions right now.

in prod, we have pinned it to version 4.0.6-r28351-1, which does work fine.

this is maybe related to https://github.com/sagemathinc/cocalc/issues/4169

in any case, this ticket is just a reminder that something started to be slightly incompatible and it's worth looking into this at some point. Best bet, something changed in version 4.1 and we should catch up...