(Voverriding_fontspec_alist): New variable.

(Voverriding_fontspec_alist): New variable.
(lookup_overriding_fontspec): New function.
(fontset_ref_via_base): Call lookup_overriding_fontspec if
necessary.
(fontset_font_pattern): Likewise.
(regulalize_fontname): New function.
(Fset_fontset_font): Call regulalize_fontname.
(Fset_overriding_fontspec_internal): New function.
(syms_of_fontset): Initialize and staticprop
Voverriding_fontspec_alist.
(syms_of_fontset): Defsubr Sset_overriding_fontspec_internal.