(Fimage_size, image_ascent): Adapt to the change of image

(Fimage_size, image_ascent): Adapt to the change of image
margins.
(lookup_image): If `:margin MARGIN' is specified, and MARGIN is a
pair of integers `(X . Y)', use X for the horizontal and Y for the
vertical margin.
(IMAGE_POSITIVE_INTEGER_VALUE_OR_PAIR): New enumerator.
(parse_image_spec): Check it.
(xbm_format, xpm_format, pbm_format, png_format, jpeg_format)
(tiff_format, gif_format, gs_format): Use
IMAGE_POSITIVE_INTEGER_VALUE_OR_PAIR for :margin.