code: function(renderer, mimetype)
  	{
  		mimetype	= mimetype	|| "image/png";
  		var dataUrl	= renderer.domElement.toDataURL(mimetype);
  		return dataUrl;
  	}
docs: Take a screenshot of a renderer
  - require WebGLRenderer to have "preserveDrawingBuffer: true" to be set
  - TODO is it possible to check if this variable is set ? if so check it
  and make advice in the console.log
  - maybe with direct access to the gl context...
  
  @param {Object} renderer to use
  @param {String} mimetype of the output image. default to "image/png"
  @param {String} dataUrl of the image