public virtual bool on_currently_loading_frame ()
Used to determine how to respond to the area_updated signal on PixbufLoader when loading an animation.
The `::area_updated` signal is emitted for an area of the frame currently streaming in to the loader. So if you're on the currently loading frame, you will need to redraw the screen for the updated area.
`TRUE` if the frame we're on is partially loaded, or the last frame