A first step towards abandoning the CoglObject type system: convert
CoglFramebuffer, CoglOffscreen and CoglOnscreen into GObjects.
CoglFramebuffer is turned into an abstract GObject, while the two others
are currently final. The "winsys" and "platform" are still sprinkled
'void *' in the the non-abstract type instances however.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1496
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.
