Always shape at size=upem

Fixes bug with uniscribe not handling GIGANTIC sizes.
diff --git a/util/options.cc b/util/options.cc
index aab2403..4e75827 100644
--- a/util/options.cc
+++ b/util/options.cc
@@ -494,7 +494,7 @@
   font = hb_font_create (face);
 
   unsigned int upem = hb_face_get_upem (face);
-  hb_font_set_scale (font, font_size * upem, font_size * upem);
+  hb_font_set_scale (font, upem, upem);
   hb_face_destroy (face);
 
 #if HAVE_FREETYPE