By sealing for future writes, we broke Wayland SHM support. It seems like the
wayland library maps the SHM in read/write mode. This is visible through no
display and an error message like this:
wl_shm@7: error 2: failed mmap fd 43: Operation not permitted
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5684>
There is no guarantee that g_get_user_runtime_dir() is in a tmpfs. Using
an explicit shared memory API seems safer for all POSIX platforms.
Note that Android does not have shm_open() and only added memfd_create()
since API level 30 (Android 11).
Sponsored-by: Netflix Inc.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5328>
