(x_build_heuristic_mask): Accept a list `(R G B)'

(x_build_heuristic_mask): Accept a list `(R G B)'
as background color specification instead of an integer.
(image-cache-eviction-delay): Replaces image-eviction-seconds.
(Vimage_cache_eviction_delay): Replaces Vimage_eviction_seconds.
(clear_image_cache, syms_of_xfns): Use it.
(Qpostscript): Replaces Qghostscript.
(gs_type): Use it.
(gs_image_p): Ditto.
(syms_of_xfns): Initialize Qpostscript.