Add an optional ArdourCanvas::Item::prepare_for_render interface

Called when an item has requested a redraw and intersects with visible
canvas area.

Also add Canvas::prepare_for_render that will call Item::prepare_for_render for
items visible on the canvas.

libs/canvas/canvas/canvas.h

@@ -87,6 +87,8 @@ public:
 
 	void render (Rect const &, Cairo::RefPtr<Cairo::Context> const &) const;
 
+	void prepare_for_render (Rect const &) const;
+
 	/** @return root group */
 	Item* root () {
 		return &_root;
@@ -214,6 +216,8 @@ public:
 		Canvas::render (rect, ctx);
 	}
 
+	void prepare_for_render () const;
+
 	uint32_t background_color() { return Canvas::background_color (); }
 
 protected:

libs/canvas/canvas/container.h

@@ -53,6 +53,12 @@ public:
 	 *  (just call Item::render_children()). It can be overridden as necessary.
 	 */
 	void render (Rect const & area, Cairo::RefPtr<Cairo::Context> context) const;
+
+	/** The prepare_for_render() method is likely to be identical in all
+	 * containers (just call Item::prepare_for_render_children()). It can be
+	 * overridden as necessary.
+	 */
+	void prepare_for_render (Rect const & area) const;
 };
 
 }

libs/canvas/canvas/item.h

@@ -75,6 +75,13 @@ public:
 	 */
 	virtual void render (Rect const & area, Cairo::RefPtr<Cairo::Context>) const = 0;
 
+	/** Item has changed will be rendered in next render pass so give item a
+	 * chance to perhaps schedule work in another thread etc.
+	 *
+	 *  @param area Area to draw, in **window** coordinates
+	 */
+	virtual void prepare_for_render (Rect const & area) const { }
+
 	/** Adds one or more items to the vector @param items based on their
 	 * covering @param point which is in **window** coordinates
 	 *
@@ -309,6 +316,7 @@ protected:
 
 	void add_child_bounding_boxes (bool include_hidden = false) const;
 	void render_children (Rect const & area, Cairo::RefPtr<Cairo::Context> context) const;
+	void prepare_for_render_children (Rect const & area) const;
 
 	Duple scroll_offset() const;
 	Duple position_offset() const;