SteamVR_Render.cs workaround to allow arbitrary 3rd controller

SteamVR_Render_edit.cs

void Awake()
{
#if (UNITY_5_3 || UNITY_5_2 || UNITY_5_1 || UNITY_5_0)
    var go = new GameObject("cameraMask");
    go.transform.parent = transform;
    cameraMask = go.AddComponent<SteamVR_CameraMask>();
#endif
    if (System.IO.File.Exists(externalCameraConfigPath)) {
        {
            if (externalCamera == null) {
                var prefab = Resources.Load<GameObject>("SteamVR_ExternalCamera");
                var instance = Instantiate(prefab);
                instance.gameObject.name = "External Camera";
            }
            externalCamera = instance.transform.GetChild(0).GetComponent<SteamVR_ExternalCamera>();
            externalCamera.configPath = externalCameraConfigPath;
            externalCamera.ReadConfig();
        }
    }