toDataURL	= function(renderer, mimetype)
	{
		mimetype	= mimetype	|| "image/png";
		var dataUrl	= renderer.domElement.toDataURL(mimetype);
		return dataUrl;
	}
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