[font] Update multipliers when creating sub_font

Fixes https://github.com/harfbuzz/harfbuzz/issues/1910

src/hb-font.cc

@@ -1385,6 +1385,7 @@ hb_font_create_sub_font (hb_font_t *parent)
 
   font->x_scale = parent->x_scale;
   font->y_scale = parent->y_scale;
+  font->mults_changed ();
   font->x_ppem = parent->x_ppem;
   font->y_ppem = parent->y_ppem;
   font->ptem = parent->ptem;