Headless sharing and X11 screen id on 0.29.1

[strohel]

PR just for documentation of patches used. Relates to https://github.com/tonarino/portal/pull/2741

[mbernat]

glutin 0.31 has removed most windowing code (which should now be provided by other libraries instead), so this PR might not be necessary anymore, once we upgrade.

• The only usage of initWithFormat_shareContext seems to be during context creation and it uses the provided shared context (see code).
• As mentioned on the winit fork, screen_id should now be handled by the new versions of winit and raw-window-handle crates, so might just work now without doing anything in glutin.

Remains to be tested.