Instead of passing around an X11 Display pointer that is retrieved from
the default Gdk backend, then finding the MetaX11Display from said X11
Display, pass the MetaX11Display directly.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/854
The functionality core/core.c and core/core.h provides are helpers for
the window decorations. This was not possible to derive from the name
itself, thus rename it and put it in the right place.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/854