One more minor cleanup of font subsystem.

One more minor cleanup of font subsystem.
* font.h (struct font_driver): Convert text_extents to
return void because returned value is never actually used.
* macfont.c (macfont_text_extents):
* w32font.c (w32font_text_extents):
* xftfont.c (xftfont_text_extents): Adjust to return void
and assume that 'metrics' argument is always non-NULL.
* ftfont.c (ftfont_text_extents):
* xfont.c (xfont_text_extents): Likewise.  Avoid redundant memset.