widget: Make invalidation coordinates widget-relative

widget: Make invalidation coordinates widget-relative

Consider the coordinates passed to gtk_widget_queue_draw_region to be
relative to @widget's origin, not its parent. That implies passing
priv->allocation or priv->clip to _queue_draw_region of a widget means
using its parent as @widget.

This fixes GtkScrolledWindow overshoot invalidation, which assumed the
coordinates to be widget-relative and not parent-relative.