Make meson link libmutter using -fvisibility=hidden, and introduce META_EXPORT
and META_EXPORT_TEST defines to mark a symbols as visible.
The TEST version is meant to be used to flag symbols that are only used
internally by mutter tests, but that should not be considered public API.
This allows us to be more precise in selecting what is exported and what is
not, without the need of a version-script file that would be more complicated
to maintain.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/395
The elementary guys would like this as an API, and I don't see any
reason to refuse -- this is quite nice shadow painting code :)
For some reason, gobject-introspection can't seem to cope with
MetaWindowShape. I'll look into it a bit later, but for now, mark
the function it has trouble with as (skip).
