Add GetLocalPosition to controls (#4872)

* Add GetLocalPosition to controls

In my case I want the mouse's position inside of the control to show something under it unless there's a better way.

* weh

---------

Co-authored-by: Pieter-Jan Briers <pieterjan.briers+git@gmail.com>

Robust.Client/UserInterface/Control.Layout.cs

@@ -468,6 +468,14 @@ namespace Robust.Client.UserInterface
             }
         }
 
+        /// <summary>
+        /// Gets the screen coordinates position relative to the control.
+        /// </summary>
+        public Vector2 GetLocalPosition(ScreenCoordinates coordinates)
+        {
+            return coordinates.Position - GlobalPixelPosition;
+        }
+
         /// <summary>
         /// Notify the layout system that this control's <see cref="Measure"/> result may have changed
         /// and must be recalculated.