pressure-vessel: Preserve X11 display number instead of redirecting it to :99

Suppose the user's "real" X11 display on the host is Xorg or Xwayland listening on :42, but they also have an Xvfb server listening on :99.

If we change the X11 display number to the arbitrary value :99, and the Flatpak sandbox shares its network namespace with the host, then clients inside the Flatpak sandbox will prefer to connect to the abstract socket @/tmp/.X11-unix/X99 (which is Xvfb), rather than the filesystem-backed socket /tmp/.X11-unix/X99 in the sandbox (which is really /tmp/.X11-unix/X42 on the host, i.e. Xorg or Xwayland).

If they're relying on Xauthority (MIT-MAGIC-COOKIE-1) for access control (as many display managers do), then this will fail, because we gave the sandboxed app access to the cookies for Xorg/Xwayland (rewriting their display number from 42 to 99 as we did so), but Xvfb does not accept those cookies.

If we're relying on xhost +"si:localuser:$(id -nu)" for access control (as gdm does), then the Flatpak app will successfully (!) connect to whatever is on :99, for example Xvfb or Xephyr, which is rarely what anyone wants either.

Resolves: https://github.com/flatpak/flatpak/issues/3357
Forwarded: https://github.com/flatpak/flatpak/pull/5034
Resolves: https://github.com/ValveSoftware/steam-runtime/issues/524
Signed-off-by: Simon McVittie smcv@collabora.com