handle enter/leave items when zooming and scrolling occur

libs/canvas/canvas.cc

@@ -49,6 +49,14 @@ Canvas::scroll_to (Coord x, Coord y)
 {
 	_scroll_offset_x = x;
 	_scroll_offset_y = y;
+
+	enter_leave_items (0); // no current mouse position 
+}
+
+void
+Canvas::zoomed ()
+{
+	enter_leave_items (0); // no current mouse position
 }
 
 /** Render an area of the canvas.

libs/canvas/canvas/canvas.h

@@ -107,6 +107,8 @@ public:
         void scroll_to (Coord x, Coord y);
         virtual Rect visible_area () const = 0;
 
+        void zoomed();
+    
         std::string indent() const;
         std::string render_indent() const;
         void dump (std::ostream&) const;