(Qcenter): New variable.

(Qcenter): New variable.
(enum image_value_type): Add IMAGE_ASCENT_VALUE.
(parse_image_spec): Handle IMAGE_ASCENT_VALUE.
(image_ascent): New function.
(lookup_image): Recognize `:ascent center'.
(xbm_format, xpm_format, pbm_format, png_format, jpeg_format)
(tiff_format, gif_format, gs_format): Use IMAGE_ASCENT_VALUE.
(xbm_load): Don't set image's ascent here.
(xbm_image_p, xpm_image_p, pbm_image_p, png_image_p)
(jpeg_image_p, tiff_image_p, gif_image_p, gs_image_p): Don't
check ascent values here.
(Fimagep, Flookup_image [GLYPH_DEBUG]: Removed.
(syms_of_xfns) [GLYPH_DEBUG]: Don't defsubr removed functions.
(syms_of_xfns): Initialize Qcenter.