Xpra: Ticket #2712: all servers should expose opengl capabilities

Not just X11 servers, the code is generic already and just needs to be moved to the display mixin.

It can be useful to see the server's GPU information without having to connect the GUI and run a separate tool.

Mon, 06 Apr 2020 08:55:51 GMT - totaamwin32: status changed; resolution set

Done in r25987 + r25988 + r25991.

Sat, 23 Jan 2021 05:58:45 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2712