Fix growMargin() not returning modified Rect2/Rect2i

(cherry picked from commit 0ac6f4c40b42e67439c15af8d6a51ff1889b0f15)

modules/mono/glue/GodotSharp/GodotSharp/Core/Rect2.cs

@@ -127,7 +127,7 @@ namespace Godot
         {
             var g = this;
 
-            g.GrowIndividual(Margin.Left == margin ? by : 0,
+            g = g.GrowIndividual(Margin.Left == margin ? by : 0,
                     Margin.Top == margin ? by : 0,
                     Margin.Right == margin ? by : 0,
                     Margin.Bottom == margin ? by : 0);