Recalculate default font when switching font backend

Recalculate default font when switching font backend

This is an updated version of the patch by Dmitry Antipov
<dmantipov@yandex.ru> in
<https://debbugs.gnu.org/cgi/bugreport.cgi?bug=23386#43>.

Fixes Bug#23386

* src/dispextern.h (struct redisplay_interface): New member
default_font_parameter.
* src/xterm.h: Add prototype for x_default_font_parameter.
* src/xterm.c (x_redisplay_interface): Initialize
default_font_parameter member.
* src/xfns.c (x_default_font_parameter): Make non-static.
* src/w32term.h: Add prototype for w32_default_font_parameter
* src/w32fns.c (w32_default_font_parameter): Make non-static.
* src/w32term.c (w32_redisplay_interface): Initialize
default_font_parameter member.
* src/nsterm.m (ns_redisplay_interface): Add dummy
ns_default_font_parameter (there is currently only one possible font
backend on macOS).  Initialize default_font_parameter member.
* src/frame.c (gui_set_font_backend): Recalculate default font using
RIF default_font_parameter to avoid crash when changing font backend.