st-theme-node: Add method to invalidate drawing state

StThemeNode caches its resources aggressively to keep the required
work on paint to a minimum - right now, resources are only recreated
on allocation changes.
In order to update the background-image property correctly when the
underlying file changes, resources need to be recreated without a
size change, so add an explicit method for that.

https://bugzilla.gnome.org/show_bug.cgi?id=679268