Use "origin" instead of "o" in Transform2D.Rotation

modules/mono/glue/Managed/Files/Transform2D.cs

@@ -17,7 +17,7 @@ namespace Godot
 
         public real_t Rotation
         {
-            get { return Mathf.Atan2(y.x, o.y); }
+            get { return Mathf.Atan2(y.x, origin.y); }
         }
 
         public Vector2 Scale