/* 
 * Draw a rectangle outline at the specified coordinates.
 *
 * Examples:
 *   # draw rectangle around edge of image using context color
 *   rect = [1, 1, im.width - 2, im.height - 2]
 *   im.draw_rect rect
 *
 *   # draw magenta rectangle outline in top-left corner of image
 *   color = Imlib2::Color::MAGENTA
 *   im.draw_rect [0, 0], [im.width / 2, im.height / 2], color
 *
 *   # draw square from 10, 10 to 30, 30 using context color
 *   im.draw_rect [10, 10, 20, 20] 
 *
 */
static VALUE image_draw_rect(int argc, VALUE *argv, VALUE self) {