(four_corners_best): New arg CORNERS specifies what pixels

(four_corners_best): New arg CORNERS specifies what pixels
to look at in case image has margin.
(x_create_bitmap_mask): Pass NULL for CORNERS to four_corners_best.
(image_background, image_background_transparent)
(x_build_heuristic_mask): Pass img->corners to four_corners_best.
(gif_load): Set img->corners according to image's margin spec.
Use img->corners values directly where applicable.
Save image extension data in img->data.lisp_val.
(gif_clear_image): New function to free img->data.lisp_val.
(gif_type): Use it instead of generic x_clear_image.
(Fimage_extension_data): New defun.
(syms_of_image): Defsubr it.