fix(UIPointer): ensure event input pointers list is managed correctly

Previously, whenever a UIPointer was initialised it would get added to
the Event Input pointers list, but it could end up being added multiple
times to the same list as no checking was done to see if it already
existed within the list and it was also not removed from the list when
the UIPointer was disabled.

This fix ensures the list is correctly managed and also caches the
event system and event system input to prevent them from being re-found
each time a pointer is enabled.

Assets/VRTK/Scripts/Internal/VRTK_EventSystemVRInput.cs

@@ -7,11 +7,11 @@
 
     public class VRTK_EventSystemVRInput : PointerInputModule
     {
-        public List<VRTK_UIPointer> pointers;
+        public List<VRTK_UIPointer> pointers = new List<VRTK_UIPointer>();
 
         public void Initialise()
         {
-            pointers = new List<VRTK_UIPointer>();
+            pointers.Clear();
         }
 
         public override void Process()

Assets/VRTK/Scripts/UI/VRTK_UIPointer.cs

@@ -120,6 +120,9 @@ public enum ClickMethods
         private bool lastPointerPressState = false;
         private bool lastPointerClickState = false;
 
+        private EventSystem cachedEventSystem;
+        private VRTK_EventSystemVRInput cachedEventSystemInput;
+
         private const string ACTIVATOR_FRONT_TRIGGER_GAMEOBJECT = "UIPointer_Activator_Front_Trigger";
 
         public virtual void OnUIPointerElementEnter(UIPointerEventArgs e)
@@ -317,20 +320,46 @@ private void OnEnable()
             controllerRenderModel = VRTK_SDK_Bridge.GetControllerRenderModel(controller.gameObject);
         }
 
+        private void OnDisable()
+        {
+            if (cachedEventSystemInput && cachedEventSystemInput.pointers.Contains(this))
+            {
+                cachedEventSystemInput.pointers.Remove(this);
+            }
+        }
+
         private void ConfigureEventSystem()
         {
-            var eventSystem = FindObjectOfType<EventSystem>();
-            var eventSystemInput = SetEventSystem(eventSystem);
+            if (!cachedEventSystem)
+            {
+                cachedEventSystem = FindObjectOfType<EventSystem>();
+            }
+
+            if (!cachedEventSystemInput)
+            {
+                cachedEventSystemInput = SetEventSystem(cachedEventSystem);
+            }
+
+            if (cachedEventSystem && cachedEventSystemInput)
+            {
+                if (pointerEventData == null)
+                {
+                    pointerEventData = new PointerEventData(cachedEventSystem);
+                }
+
+                StartCoroutine(WaitForPointerId());
 
-            pointerEventData = new PointerEventData(eventSystem);
-            StartCoroutine(WaitForPointerId());
-            eventSystemInput.pointers.Add(this);
+                if (!cachedEventSystemInput.pointers.Contains(this))
+                {
+                    cachedEventSystemInput.pointers.Add(this);
+                }
+            }
         }
 
         private IEnumerator WaitForPointerId()
         {
             var index = (int)VRTK_SDK_Bridge.GetControllerIndex(controller.gameObject);
-            while(index < 0 || index == int.MaxValue)
+            while (index < 0 || index == int.MaxValue)
             {
                 index = (int)VRTK_SDK_Bridge.GetControllerIndex(controller.gameObject);
                 yield return null;