Canvas: modify Arc to work with layout

libs/canvas/arc.cc

@@ -146,3 +146,23 @@ Arc::covers (Duple const & point) const
 		(angle_degs <= (_start_degrees + _arc_degrees)) &&
 		(radius < _radius);
 }
+
+void
+Arc::size_allocate (Rect const & r)
+{
+	begin_change ();
+
+	Item::size_allocate (r);
+
+	/* This is an arc - some section of a circle, so any difference between
+	 * height and width cannot change what is drawn. Pick width arbitrarily
+	 * as the "key" dimension
+	 */
+
+	_radius = min (r.width(),r.height()) / 2.0;
+	_center = Duple ((r.width()/2.), (r.height() /2.));
+
+	_bounding_box_dirty = true;
+
+	end_change ();
+}

libs/canvas/canvas/arc.h

@@ -37,6 +37,8 @@ public:
 	void render (Rect const & area, Cairo::RefPtr<Cairo::Context>) const;
 	void compute_bounding_box () const;
 
+	void size_allocate (Rect const & r);
+
 	void set_center (Duple const &);
 	void set_radius (Coord);
 	void set_arc (double degrees);