Support font driver supersession

Support font driver supersession

* src/font.c (font_update_drivers): If argument NEW_DRIVERS is t, then don't
use superseded drivers.
(syms_of_font) <Qfont_driver_superseded_by>: New DEFSYM.

* src/ftfont.c (syms_of_ftfont) [HAVE_HARFBUZZ]:
* src/ftcrfont.c (syms_of_ftcrfont) [HAVE_HARFBUZZ]:
* src/w32font.c (syms_of_w32font) [HAVE_HARFBUZZ]:
* src/xftfont.c (syms_of_xftfont) [HAVE_HARFBUZZ]: Make Harfbuzz variants
supersede non-Harfbuzz ones.

* src/w32fns.c (Fx_create_frame, w32_create_tip_frame): Remove font backend
determination code.