code: function(engine) {
      if (typeof GFX_ENGINES[engine] === 'undefined') {
        return grunt.fail.warn('Invalid render engine specified');
      }
      grunt.verbose.ok('Using render engine: ' + GFX_ENGINES[engine].name);
  
      if (engine === 'im') {
        return gm.subClass({ imageMagick: (engine === 'im') });
      }
  
      return gm;
    }
docs: Set the engine to ImageMagick or GraphicsMagick
  
  @private
  @param  {string}          engine     im for ImageMagick, gm for GraphicsMagick