(Vimage_types): Move from xdisp.c.

(Vimage_types): Move from xdisp.c.
(Vimage_type_cache): New variable.
(define_image_type): New argument indicating whether an image library was
loaded; cache loaded status and return t on success, nil otherwise.
(CACHE_IMAGE_TYPE, ADD_IMAGE_TYPE): New macros.
(w32_delayed_load): New function to load an image library from a list of
possible filenames.
(init_xpm_functions, init_png_functions, init_jpeg_functions)
(init_tiff_functions, init_gif_functions): Use `w32_delayed_load'.
(CHECK_LIB_AVAILABLE): Call `define_image_library' with new argument.
(Finit_image_library): New function, extracted from `init_image'.  Try to
initialize an image library on demand and cache whether we were successful
or not.
(syms_of_image): Initialize `Vimage_types' and `Vimage_type_cache'.  Add
recognized image types to Vimage_types. Export `init-image-library'.
(init_image): Remove initialization of all image types, except xbm and pbm.