While each stage has at most a GdkFrameClock, the same GdkFrameClock
instance may drive multiple stages per frame. This means that the
mapping between a GdkFrameClock and a ClutterStage is a 1:M one, not a
1:1.
We should store a list of stages associated to each frame clock
instance, so that we can iterate over it when we need to update the
stages.
This commit fixes redraws of applications using multiple stages,
especially when using clutter-gtk.
4f8643cea3