DPS font backend uses the wrong font

[Po Lu <luangruo@yahoo.com>]

As of 2028df78, font backends are required to use s->font instead of
s->face->font, even in the NS port, because s->face is now set to mouse
face by redisplay independently of terminal specific code.

Every font backend except for nsfont already does that, so it would make
sense to change nsfont to do it correctly as well.  But is there
something I'm missing here?

IOW, I'm suggesting to change the following snippet of nsfont_draw:

  font = (struct nsfont_info *)s->face->font;
  if (font == NULL)
    font = (struct nsfont_info *)FRAME_FONT (s->f);

to

  font = (struct nsfont_info *)s->font;
  if (font == NULL)
    font = (struct nsfont_info *)FRAME_FONT (s->f);

Thanks.