(gif_load): Avoid sign extension and thus out of bounds

(gif_load): Avoid sign extension and thus out of bounds
color indices when accessing raster pixels.
(gif_image_p, png_image_p, jpeg_image_p, tiff_image_p): Allow only
one of :file or :data.
(enum pbm_keyword_index): Add PBM_DATA.
(pbm_format): Add :data.
(pbm_image_p): Allow either :file or :data.
(pbm_read_file): New function.
(pbm_scan_number): Rewritten to read from string.
(pbm_load): Support :data.