Commit 25f416c13d added additional compilation warnings, including
-Werror=return-type. There are several places where this results
in build failures if `g_assert_not_reached()` is disabled at compile
time and the compiler misses a return value.
https://gitlab.gnome.org/GNOME/mutter/issues/447
In plenty of places a non-static function was defined but didn't have
the corresponding declaration. Fix this by adding them, or alternatively
making them static.
