This ensures that two widgets sharing the same theme node won't trample
on each other's prerendered materials if the actors are of different
sizes. This also tries to be very careful to share as much as possible
during a transition.

This has the side effect that if a widget changes state a bunch of times,
we won't cache every state. Since we expect that state changes are
infrequent and that most cases we'll be able to use the texture cache
to do most of the heavy lifting, this cost is much more insignificant
than rendering a number of different actors with the same theme node
and different sizes.

https://bugzilla.gnome.org/show_bug.cgi?id=697274