StyleContext.pushAnimatableRegion

Pushes an animatable region, so all further gtk_render_*() calls between this call and the following [gtk.style_context.StyleContext.popAnimatableRegion] will potentially show transition animations for this region if [gtk.style_context.StyleContext.notifyStateChange] is called for a given state, and the current theme/style defines transition animations for state changes.

The region_id used must be unique in context so the themes can uniquely identify rendered elements subject to a state transition.