Add introspection annotations for (out) arguments.

Mark that the the third argument to meta_rectangle_union and
meta_rectangle_intersect is an (out) argument that should be created
by the language binding.

https://bugzilla.gnome.org/show_bug.cgi?id=645247

src/core/boxes.c

@@ -179,6 +179,16 @@ meta_rectangle_area (const MetaRectangle *rect)
   return rect->width * rect->height;
 }
 
+/**
+ * meta_rectangle_intersect:
+ * @src1: a #MetaRectangle
+ * @src2: another #MetaRectangle
+ * @dest: (out callee-allocates): an empty #MetaRectangle, to be filled
+ *   with the coordinates of the intersection.
+ *
+ * Returns: TRUE is some intersection exists and is not degenerate, FALSE
+ *   otherwise.
+ */
 gboolean
 meta_rectangle_intersect (const MetaRectangle *src1,
 			  const MetaRectangle *src2,
@@ -226,6 +236,13 @@ meta_rectangle_equal (const MetaRectangle *src1,
           (src1->height == src2->height));
 }
 
+/**
+ * meta_rectangle_union:
+ * @rect1: a #MetaRectangle
+ * @rect2: another #MetaRectangle
+ * @dest: (out callee-allocates): an empty #MetaRectangle, to be filled
+ *   with the coordinates of the bounding box.
+ */
 void
 meta_rectangle_union (const MetaRectangle *rect1,
                       const MetaRectangle *rect2,