boxes: Add helper to scale rectangles by a double

And change the similar region scaling helper to use this one.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/362

src/core/boxes-private.h

@@ -39,6 +39,12 @@ typedef enum
   FIXED_DIRECTION_Y    = 1 << 1,
 } FixedDirections;
 
+typedef enum _MetaRoundingStrategy
+{
+  META_ROUNDING_STRATEGY_SHRINK,
+  META_ROUNDING_STRATEGY_GROW,
+} MetaRoundingStrategy;
+
 /* Output functions -- note that the output buffer had better be big enough:
  *   rect_to_string:   RECT_LENGTH
  *   region_to_string: (RECT_LENGTH+strlen(separator_string)) *
@@ -219,6 +225,11 @@ GList* meta_rectangle_find_nonintersected_monitor_edges (
 gboolean meta_rectangle_is_adjecent_to (MetaRectangle *rect,
                                         MetaRectangle *other);
 
+void meta_rectangle_scale_double (const MetaRectangle  *rect,
+                                  double                scale,
+                                  MetaRoundingStrategy  rounding_strategy,
+                                  MetaRectangle        *dest);
+
 static inline ClutterRect
 meta_rectangle_to_clutter_rect (MetaRectangle *rect)
 {