x11 client crashes when the display does not have randr support

[Antoine Martin]

(gdb) py-bt
Traceback (most recent call first):
  File "/usr/lib64/python3.7/site-packages/xpra/platform/xposix/gui.py", line 369, in get_vrefresh
    v = randr.get_vrefresh()
  File "/usr/lib64/python3.7/site-packages/xpra/client/ui_client_base.py", line 207, in init_ui
    v = get_vrefresh()
  File "/usr/lib64/python3.7/site-packages/xpra/scripts/main.py", line 1583, in get_client_app
    app.init_ui(opts)
  File "/usr/lib64/python3.7/site-packages/xpra/scripts/main.py", line 1430, in run_client
    app = get_client_app(error_cb, opts, extra_args, mode)
  File "/usr/lib64/python3.7/site-packages/xpra/scripts/main.py", line 447, in run_mode
    return run_client(error_cb, options, args, mode)
  File "/usr/lib64/python3.7/site-packages/xpra/scripts/main.py", line 115, in main
    return run_mode(script_file, err, options, args, mode, defaults)
  File "/usr/bin/xpra", line 9, in <module>
    sys.exit(main(sys.argv[0], sys.argv))
(gdb) q

get_vrefresh() calls XRRGetScreenInfo and we shouldn't call it on a system that doesn't have randr.
The server already handles screen without randr gracefully.

[Antoine Martin]

Fixes:

• r25147 for client
• r25148 for server

Backports in r25149 + r25150.