Xpra: Ticket #2520: warn users when shadow screen is blank

This happens with virtualbox.

Thu, 19 Dec 2019 09:42:27 GMT - Antoine Martin: status changed; resolution set

Done in r24766.

Tue, 31 Mar 2020 10:53:18 GMT - Antoine Martin:

Also: r25842 for detecting wayland displays, which cannot be captured.

Sat, 23 Jan 2021 05:53:30 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2520