The current way we keep track of skip-paint children is more
difficult than it needs to be, and can lead to subtle bugs.
(For one, the skip-paint state of a child is remembered
when it is removed then added back again, which is completely
unexpected.)
Instead of using weak references to track children, just remove
items from the skip-paint list by overriding the remove() virtual
function of the ClutterContainer interface.
The 'skip_paint' hash table is then destroyed in finalize rather than
dispose since it doesn't hold references to memory any more but just
passively tracks an attribute of the children that are currently in
the container.
https://bugzilla.gnome.org/show_bug.cgi?id=608848
Ideally we'd be able to override _paint, but given that we can't
at the moment, this method gives a way to implement containers
which don't happen to paint all of their children.
https://bugzilla.gnome.org/show_bug.cgi?id=603522
Subclass ClutterGroup (to avoid having to implement all of dispose,
raise, lower, add, etc.), and have it proxy the allocation requests
out into signals. We have to group up the two out parameters
into a struct unfortunately.
Included example code in the C file source for now.
