fix(SnapDropZone): ensure valid object check is done at correct point

The valid object check was being done after a touched object was added
to the valid snap object list, meaning that any object that was not
valid but touching the object would cause the highlighter to display
even if the object was not valid.

Assets/VRTK/Prefabs/SnapDropZone/VRTK_SnapDropZone.cs

@@ -401,10 +401,10 @@ protected virtual void OnTriggerExit(Collider collider)
 
         protected virtual void CheckCanSnap(VRTK_InteractableObject interactableObjectCheck)
         {
-            if (interactableObjectCheck != null)
+            if (interactableObjectCheck != null && ValidSnapObject(interactableObjectCheck, true))
             {
                 AddCurrentValidSnapObject(interactableObjectCheck);
-                if (!isSnapped && ValidSnapObject(interactableObjectCheck, true))
+                if (!isSnapped)
                 {
                     ToggleHighlight(interactableObjectCheck, true);
                     interactableObjectCheck.SetSnapDropZoneHover(this, true);