It already was built into it without any symbols exported, but also
duplicated in test cases that used it. Make it so that the built in
functions are exported, with prefixes, and make all tests use the
exported functions. While at it, make things go via MetaContext or
MetaBackend depending on how early in initialization things are run.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2262>
