(xlfd_parse_name): Add const to the arg name. Call

(xlfd_parse_name): Add const to the arg name.  Call
mfont__set_property instead of mfont_set_spec.  Set font->spacing.
(xlfd_unparse_name): Handle spacing.
(mfont__init): Initialize new variables.
(mfont__id): Handle spacing.
(mfont__merge): Likewise.
(mfont__set_spec): This function deleted.
(mfont__parse_name_into_font): Add const to the arg name.  Fix the
condition of calling mfont__ft_parse_name.
(Mspacing, Mascent, Mdescent, Mmax_advance): New variables.
(mfont_get_prop): Handle properties spacing, ascent, descent,
and max-advance.
(mfont_find): Return a realized font.
(mfont_from_name): Add const to the arg name.