/* 
 * Fill a rectangle at the specified coordinates.
 *
 * Examples:
 *   # fill image using context color
 *   rect = [0, 0, im.width, im.height]
 *   im.fill_rect rect
 *
 *   # fill top-left quarter of image with green
 *   color = Imlib2::Color::GREEN
 *   im.fill_rect [0, 0], [im.width / 2, im.height / 2], color
 *
 *   # fill square from 10, 10 to 30, 30 using context color
 *   im.fill_rect [10, 10, 20, 20] 
 *
 */
static VALUE image_fill_rect(int argc, VALUE *argv, VALUE self) {