Returns a data: URL containing a representation of the image in the format specified by type (defaults to PNG). The returned image is 96dpi.
If the height or width of the canvas is 0, "data:," representing the empty string, is returned.
If the type requested is not image/png, and the returned value starts with data:image/png, then the requested type is not supported.
Chrome supports the image/webp type.
If the requested type is image/jpeg or image/webp, then the second argument, if it is between 0.0 and 1.0, is treated as indicating image quality;
if the second argument is anything else, the default value for image quality is used. Other arguments are ignored.