/*
 * Save an Imlib2::Image to a file (throws an exception on error).
 * 
 * Examples:
 *   image.save 'output_file.png'
 *
 *   filename = 'output_file.jpg'
 *   begin
 *     image.save filename
 *   rescue Imlib2::FileError
 *     $stderr.puts "Couldn't save file \"#{filename}\": " + $!
 *   end
 *
 */
static VALUE image_save(VALUE self, VALUE val) {