The ro_anonymous_file is an abstraction around anonymous read-only files.
os_ro_anonymous_file_create creates such a file with the contents passed
in data. Subsequent calls to os_ro_anonymous_file_get_fd return a fd
that's ready to be send over the socket.
When mapmode is RO_ANONYMOUS_FILE_MAPMODE_PRIVATE the fd is only
guaranteed to be mmap-able readonly with MAP_PRIVATE but does not
require duplicating the file for each resource when memfd_create is
available. RO_ANONYMOUS_FILE_MAPMODE_SHARED may be used when the client
must be able to map the file with MAP_SHARED but it also means that the
file has to be duplicated even when memfd_create is available.

See also:
weston commit 76829fc4


wayland commit 905c0a341ddf0a885811d19e2b79c65a3f1d210c

Signed-off-by: Sebastian Wick <sebastian@sebastianwick.net>