destroy() checks if we are visible to decide whether to destroy the
actor immediately or after a fade out animation. Since actors are
visible by default it would always end up destroying only after the
animation time.
https://bugzilla.gnome.org/show_bug.cgi?id=691963
Due to limitations and bugs in SpiderMonkey's GC, wrapper objects
for cairo contexts and similar may not get cleaned up immediately
after repainting, leading to leaking memory. Explicitly disposing
of such objects after they're not needed can clean up large portions
of memory for cairo surfaces.
https://bugzilla.gnome.org/show_bug.cgi?id=685513
