Introduce TrackablePointer.movementDuration

docs/api/trackable-pointer.md

 
 How much this pointer has moved, in space units, since its tracking began. You can get the average speed of motion by calculating the ratio `movementLength / duration`.
 
+### movementDuration
+
+`pointer.movementDuration: number, read-only`
+
+The total time, in seconds, in which this pointer has moved. You can get the average speed of motion, excluding the times in which this pointer has not moved, by calculating the ratio `movementLength / movementDuration`.
+
 ### isPrimary
 
 `pointer.isPrimary: boolean, read-only`

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

             // determine how much this pointer has moved since its tracking began
             const movementLength = previous ? previous.movementLength + deltaPosition.length() : 0;
 
+            // determine the duration of the movement
+            const movementDuration = !previous ? 0 : previous.movementDuration + (movementLength > previous.movementLength ? deltaTime : 0);
+
             // determine whether or not this is the primary pointer for this type
             const isPrimary = event.isPrimary;
 
 
             // we create new trackable instances on each frame;
             // these will be exported and consumed by the user
-            this._newPointers.set(id, { id, phase, position, deltaPosition, initialPosition, velocity, duration, movementLength, isPrimary, kind });
+            this._newPointers.set(id, { id, phase, position, deltaPosition, initialPosition, velocity, duration, movementDuration, movementLength, isPrimary, kind });
 
         }
 

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

     /** elapsed time, in seconds, since the tracking of this pointer began */
     readonly duration: number;
 
+    /** the total time, in seconds, in which this pointer has moved */
+    readonly movementDuration: number;
+
     /** how much this pointer has moved, in space units, since its tracking began */
     readonly movementLength: number;