Fix slowdown and wrong font choosed by XSETTINGS changes. bug #5157.

Fix slowdown and wrong font choosed by XSETTINGS changes. bug #5157.

* font.c (font_open_entity): Enable chache and call cached_font_ok
for the driver if defined.
(QCuser_spec): New symbol.
(font_spec_from_name): Save name as user-spec.
(font_load_for_lface): Keep user-spec instead of name.
(font_open_by_name): Save name as user-spec.
(syms_of_font): Initialize QCuser_spec.

* xftfont.c (xftfont_open): Call xftfont_add_rendering_parameters.
(xftfont_add_rendering_parameters, xftfont_cached_font_ok): New.
(syms_of_xftfont): Initialize xftfont_driver.cached_font_ok.

* font.h (struct font_driver): Add cached_font_ok.

* font-setting.el (font-setting-change-default-font): Use user-spec
  instead of name.