Consolidate the image_cache to the terminal struct.

Consolidate the image_cache to the terminal struct.
* termhooks.h (P_): Remove redundant def.
(struct terminal): New field `image_cache'.
* frame.h (FRAME_IMAGE_CACHE): New macro.  Use it everywhere in place
of FRAME_X_IMAGE_CACHE.
* xterm.h (struct x_display_info): Remove image_cache field.
(FRAME_X_IMAGE_CACHE): Remove.  Use FRAME_IMAGE_CACHE instead.
* w32term.h (struct w32_display_info): Remove image_cache field.
(FRAME_X_IMAGE_CACHE): Remove.  Use FRAME_IMAGE_CACHE instead.
* macterm.h (struct mac_display_info): Remove image_cache field.
(FRAME_X_IMAGE_CACHE): Remove.  Use FRAME_IMAGE_CACHE instead.
* xterm.c (x_term_init):
* w32term.c (w32_term_init):
* macterm.c (mac_term_init): Set the image_cache in the terminal.
* dispextern.h (clear_image_cache, forall_images_in_image_cache):
Remove declarations.
(clear_image_caches, mark_image_cache): New declarations.
* xfaces.c (clear_face_cache):
* xdisp.c (redisplay_internal): Use clear_image_caches.
* image.c (clear_image_cache): Don't check that a frame is on
a window-system before checking if it shares the same cache.
(clear_image_caches): New function.
(Fclear_image_cache): Use it.
(mark_image): Move from allo.c.
(mark_image_cache): Move from alloc.c and forall_images_in_image_cache.
* alloc.c (mark_image, mark_image_cache): Move to image.c.
(mark_object): Don't call mark_image_cache for frames.
(mark_terminals): Call mark_image_cache.