* font.h (struct font_entity) [HAVE_NS]: New field to record

* font.h (struct font_entity) [HAVE_NS]: New field to record
font driver which was used to create this entity.
(struct font) [HAVE_WINDOW_SYSTEM]: New field to record
frame where the font was opened.
(font_close_object): Add prototype.
* font.c (font_make_entity) [HAVE_NS]: Zero out driver field.
(font_close_object): Not static any more.  Lost frame arg.
Adjust comment and users.
* alloc.c (cleanup_vector): Call font_close_object to adjust
per-frame font counters correctly.  If HAVE_NS, also call
driver-specific cleanup for font-entity objects.
* ftfont.c (ftfont_open):
* nsfont.m (nsfont_open):
* w32font.c (w32font_open_internal):
* xfont.c (xfont_open):
* xftfont.c (xftfont_open): Save frame pointer in font object.
* macfont.m (macfont_open): Likewise.
(macfont_descriptor_entity): Save driver pointer to be able
to call its free_entity routine when font-entity is swept.
* ftxfont.c (ftxfont_open): Add eassert because frame
pointer should be saved by ftfont_driver.open.