Default font scale to face upem

Makes for a better default and avoids nasty inheritance issues.
See mailing list thread "Default hb_font_t scale".

src/hb-font.cc

diff --git a/src/hb-font.cc b/src/hb-font.cc
index 94dbcdf..058d7ec 100644
--- a/src/hb-font.cc
+++ b/src/hb-font.cc
@@ -861,6 +861,8 @@
   font->face = hb_face_reference (face);
   font->klass = hb_font_funcs_get_empty ();
 
+  font->x_scale = font->y_scale = hb_face_get_upem (face);
+
   return font;
 }