As documented in g_once_init_enter(): "While @location has a volatile qualifier,
this is a historical artifact and the pointer passed to it should not be
volatile.". And effectively this now warns with modern glibc.
Drop the "volatile" qualifier from these static variables as it's expected.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1785>
Intended to be used to pass state from screen cast clients down the
line. The first use case will be a boolean whether a screen cast is a
plain recording or not, e.g. letting the Shell decide whether to use a
red dot as the icon, or the generic "sharing" symbol.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1377