Include window.h.

Include window.h.
(font_lispy_object): New function.
(font_prepare_composition): Check LGLYPH_FORM (g) to detect the
end of valid glyph.
(font_close_object): Fix getting (struct font *).
(font_at): New function.
(Ffont_get): If FONT is a font-object, get entity from it.
(Ffont_make_gstring): Initialize elements of glyphs with nil.
(Ffont_fill_gstring): Use macro LGSTRING_XXX and LGLYPH_XXX.  Fix
range check.
(Ffont_at): New function.
(syms_of_font): Defsubr Sfont_at.