/* 
 * Draw a pixel at the specified coordinates.
 *
 * Examples:
 *   im.draw_pixel 10, 10                      # draw using context color
 *   im.draw_pixel 10, 10, Imlib2::Color::BLUE # draw blue pixel
 *   im.draw_pixel [10, 10], Imlib2::Color::RED # draw red pixel
 *
 */
static VALUE image_draw_pixel(int argc, VALUE *argv, VALUE self) {