code: function (lightGroup) {
          var prevLightNumber = this._previousLightNumber;
          var currentLightNumber = this._lightNumber;
          // PENDING Performance
          for (var type in currentLightNumber[lightGroup]) {
              if (!prevLightNumber[lightGroup]) {
                  return true;
              }
              if (currentLightNumber[lightGroup][type] !== prevLightNumber[lightGroup][type]) {
                  return true;
              }
          }
          for (var type in prevLightNumber[lightGroup]) {
              if (!currentLightNumber[lightGroup]) {
                  return true;
              }
              if (currentLightNumber[lightGroup][type] !== prevLightNumber[lightGroup][type]) {
                  return true;
              }
          }
          return false;
      }
docs: Determine if light group is different with since last frame Used to determine whether to update shader and scene's uniforms in Renderer.render