/*
 * Fill a rectangle with the given Imlib2::Gradient at a given angle
 *
 * Examples:
 *   x, y, w, h = 10, 10, image.width - 20, image.height - 20
 *   angle = 45.2
 *   image.fill_gradient gradient, x, y, w, h, angle
 *
 *   rect = [5, 5, 500, 20]
 *   image.gradient gradient, rect, 36.8
 * 
 */
static VALUE image_fill_gradient(int argc, VALUE *argv, VALUE self) {