Add MetaAnonymousFile, an abstraction around anonymous read-only files.
Files can be created by calling meta_anonymous_file_new(), passing the
data of the file. Subsequent calls to meta_anonymous_file_open_fd()
return a fd that's ready to be sent over the socket.
When mapmode is META_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. META_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.
Pretty much all of this code was written for weston by Sebastian Wick,
see https://gitlab.freedesktop.org/wayland/weston/merge_requests/240.
Co-authored-by: Sebastian Wick <sebastian@sebastianwick.net>
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1012
