fix(Interaction): prevent spring joint when other controller attached

When an Interactable Object is grabbed, if that object has a joint on
it already causes the controller to be attached with a Spring Joint.
This causes an issue when another controller is grabbing the item
because it adds a Fixed Joint to the object causing the second grabbing
controller to have a Spring Joint added when grabbing from one hand
to the other.

The solution is to check to see if the existing joint is on a
controller and if it is then add a Fixed Joint instead.

Assets/SteamVR_Unity_Toolkit/Scripts/SteamVR_InteractGrab.cs

@@ -104,17 +104,27 @@ private void SnapObjectToGrabToController(GameObject obj)
             obj.transform.position = controllerAttachPoint.transform.position;
         }
 
-        if (obj.GetComponent<Joint>())
+        CreateJoint(obj);
+    }
+
+    private bool JointOnController(GameObject obj)
+    {
+        return (obj.GetComponent<Joint>() && obj.GetComponent<Joint>().connectedBody && obj.GetComponent<Joint>().connectedBody.gameObject.GetComponentInParent<SteamVR_InteractTouch>());
+    }
+
+    private void CreateJoint(GameObject obj)
+    {
+        if (obj.GetComponent<Joint>() && ! JointOnController(obj))
         {
             SpringJoint tempSpringJoint = obj.AddComponent<SpringJoint>();
             tempSpringJoint.spring = 500;
             tempSpringJoint.damper = 50;
             controllerAttachJoint = tempSpringJoint;
-        } else
+        }
+        else
         {
             controllerAttachJoint = obj.AddComponent<FixedJoint>();
         }
-
         controllerAttachJoint.connectedBody = controllerAttachPoint;
     }