Include window.h, frame.h, dispextern.h font.h.

Include window.h, frame.h, dispextern.h font.h.
(Vcomposition_function_table)
(get_composition_id): Don't handle COMPOSITION_WITH_GLYPH_STRING.
(gstring_hash_table, gstring_work, gstring_work_headers): New
variables.
(gstring_lookup_cache, composition_gstring_put_cache)
(composition_gstring_from_id, composition_gstring_p)
(composition_gstring_width, fill_gstring_header)
(fill_gstring_body, autocmp_chars, composition_compute_stop_pos)
(composition_reseat_it, composition_update_it)
(composition_adjust_point, Fcomposition_get_gstring): New
functions.
(syms_of_composite): Initialize gstring_hash_table, gstrint_work,
and gstring_work_headers.  DEFVAR_LISP composition-function-table.
Defsubr compostion_get_gstring.