GtkWidget: Add a vfunc for queue_draw_region

GtkWidget: Add a vfunc for queue_draw_region

Since widgets now cache drawn state we allow them to override
queue_draw_region to detect when some region of the widget
should be redrawn. For instance, if a widget draws the
background color in a pixel cache we will need to invalidate
that when the style context changes which queues a repaint.