(lookup_image): Block input while loading the image so

(lookup_image): Block input while loading the image so
that we won't get interrupted in a state where the image isn't yet
set up completely.
(xbm_load_image, xbm_load, xpm_load, xpm_load, lookup_rgb_color)
(lookup_pixel_color, x_to_xcolors, x_from_xcolors)
(x_disable_image, x_build_heuristic_mask, pbm_load, png_load)
(jpeg_load, tiff_load, gif_load, gs_load): Don't block/unblock
input.