invalidateContents
Called by implementations of GdkPaintable
to invalidate their contents.
Unless the contents are invalidated, implementations must guarantee that multiple calls of method@Gdk.Paintable.snapshot produce the same output.
This function will emit the signal@Gdk.Paintable::invalidate-contents signal.
If a @paintable reports the %GDK_PAINTABLE_STATIC_CONTENTS flag, it must not call this function.