(ALLOC_XPM_COLORS): Define if we can use Emacs' color

(ALLOC_XPM_COLORS): Define if we can use Emacs' color
allocation functions with the XPM lib.
(struct xpm_cached_color) [ALLOC_XPM_COLORS]: New structure.
(XPM_COLOR_CACHE_BUCKETS) [ALLOC_XPM_COLORS]: New macro.
(xpm_color_cache) [ALLOC_XPM_COLORS]: New variable.
(xpm_init_color_cache, xpm_free_color_cache, xpm_lookup_color)
(xpm_alloc_color, xpm_free_colors) [ALLOC_XPM_COLORS]: New
functions.
(xpm_load) [ALLOC_XPM_COLORS]: Use Emacs' own color allocation
functions, if possible, because these handle color allocation
failure more gracefully.
(Fimage_mask_p): New function.