Introduce immutable zero vectors

src/geometry/vector2.ts

  * 2D vectors
  */
 
+import { Nullable } from '../utils/utils';
+
 /** Small number */
 const EPSILON = 1e-6;
 
+/** Immutable zero vector */
+let ZERO: Nullable<Vector2> = null;
+
 // public / non-internal methods do not change the contents of the vector
 
 
 
 
 
+
     /**
      * Constructor
      */
     }
 
     /**
+     * Immutable zero vector
+     * @returns an immutable zero vector
+     */
+    static get ZERO(): Vector2
+    {
+        return ZERO || (ZERO = Object.freeze(Vector2.Zero()));
+    }
+
+    /**
      * The length of this vector
      * @returns sqrt(x^2 + y^2)
      */

src/geometry/vector3.ts

  * 3D vectors
  */
 
+import { Nullable } from '../utils/utils';
+
 /** Small number */
 const EPSILON = 1e-6;
 
+/** Immutable zero vector */
+let ZERO: Nullable<Vector3> = null;
+
 // public / non-internal methods do not change the contents of the vector
 
 
     }
 
     /**
+     * Immutable zero vector
+     * @returns an immutable zero vector
+     */
+    static get ZERO(): Vector3
+    {
+        return ZERO || (ZERO = Object.freeze(Vector3.Zero()));
+    }
+
+    /**
      * The length of this vector
      * @returns sqrt(x^2 + y^2 + z^2)
      */

src/trackers/pointer-tracker/pointer-tracker.ts

         // make all active trackables stationary
         this._updateAllTrackables({
             phase: 'stationary',
-            velocity: Vector2.Zero(),
-            deltaPosition: Vector2.Zero()
+            velocity: Vector2.ZERO,
+            deltaPosition: Vector2.ZERO
         });
 
         // want to reset?
             const position = new Vector2(relX, relY);
 
             // determine the position delta
-            const deltaPosition = !previous ? Vector2.Zero() :
+            const deltaPosition = !previous ? Vector2.ZERO :
                                   position._clone()._subtract(previous.position);
 
             // determine the initial position
         // cancel all active pointers
         this._updateAllTrackables({
             phase: 'canceled',
-            deltaPosition: Vector2.Zero(),
-            velocity: Vector2.Zero(),
+            velocity: Vector2.ZERO,
+            deltaPosition: Vector2.ZERO
         });
 
         // consume all events