(Vface_resizing_fonts): New variable.

(Vface_resizing_fonts): New variable.
(struct font_name): New member `resizing_ratio'.
(font_resizing_ratio): New function.
(split_font_name): Set font->resizing_ratio.
(better_font_p): Pay attention to font->resizing_ratio.
(build_scalable_font_name): Likewise.  Don't change POINT_SIZE,
RESX, and RESY fields.
(try_alternative_families): Try scalable fonts if
Vscalable_fonts_allowed is not Qt.
(syms_of_xfaces): Declare Vface_resizing_fonts as a Lisp variable.