(image_background, image_background_transparent)

(image_background, image_background_transparent)
(four_corners_best): New functions.
(xpm_format, png_format, jpeg_format, tiff_format, gif_format)
(gs_format): Add `:background' entry.
(lookup_image): Set IMG's background color if specified.
(pbm_load, xbm_load_image, png_load): Set IMG's background field
when appropriate.
(x_clear_image_1): Reset `background_valid' and
`background_transparent_valid' fields.
(x_build_heuristic_mask): Use IMAGE_BACKGROUND instead of
calculating it here.  Set IMG's background_transparent field.
(enum xpm_keyword_index): Add XPM_BACKGROUND.
(enum png_keyword_index): Add PNG_BACKGROUND.
(enum jpeg_keyword_index): Add JPEG_BACKGROUND.
(enum tiff_keyword_index): Add TIFF_BACKGROUND.
(enum gif_keyword_index): Add GIF_BACKGROUND.
(enum gs_keyword_index): Add GS_BACKGROUND.
(pbm_load, png_load, jpeg_load, tiff_load, gif_load):
Pre-calculate image background color where necessary.