Add VR model accessor in vtkOpenVRRenderWindow

Add accessor to get the vtkOpenVRModel associated to the specified
index.

Rendering/OpenVR/vtkOpenVRRenderWindow.h

@@ -235,6 +235,12 @@ public:
   vr::TrackedDevicePose_t &GetTrackedDevicePose(vr::TrackedDeviceIndex_t idx) {
     return this->TrackedDevicePose[idx]; };
 
+  /**
+  * Get the VRModel corresponding to the tracked device index
+  */
+  vtkOpenVRModel *GetTrackedDeviceModel(vr::TrackedDeviceIndex_t idx) {
+    return this->TrackedDeviceToRenderModel[idx]; };
+
   /**
    * Initialize the Vive to World setting and camera settings so
    * that the VR world view most closely matched the view from