iconGrid: Only animate visible icons

iconGrid: Only animate visible icons

Mutter recently added an optimization to only allocate
visible children [1]. That broke ClutterClones, but it
was subsequently fixed [2].

However, that exposed a third problem, this time with
FrequentView: visible but transparent icons, that are
not allocated by the icon grid, were cloned and animated
during the spring animation.

Only animate visible icons with opacity greater than 0.

[1] https://gitlab.gnome.org/GNOME/mutter/commit/0eab73dc
[2] https://gitlab.gnome.org/GNOME/mutter/commit/08a3cbfc

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/696