Xpra: Ticket #2230: show more details when opengl probe fails

We can do that easily only with python3's subprocess.Popen.communicate which now takes a timeout.

Then we can just parse the full opengl properties, including the error message and tell the user more precisely why the opengl probe failed.

See #2204.

Wed, 20 Mar 2019 16:06:28 GMT - Antoine Martin: status changed

r22160 may need to be reverted as it spits out some GTK warnings.

Fri, 27 Sep 2019 12:22:37 GMT - Antoine Martin: status, milestone changed; resolution set

Sat, 23 Jan 2021 05:45:40 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2230