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.
r22160 may need to be reverted as it spits out some GTK warnings.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2230