Use hb_face_get_upem() instead of face->get_upem()

Such that hmtx accelerator can be used from libharfbuzz-subset.

src/hb-ot-cbdt-table.hh

@@ -377,7 +377,7 @@ struct CBDT
   {
     inline void init (hb_face_t *face)
     {
-      upem = face->get_upem();
+      upem = hb_face_get_upem (face);
 
       cblc_blob = Sanitizer<CBLC>().sanitize (face->reference_table (HB_OT_TAG_CBLC));
       cbdt_blob = Sanitizer<CBDT>().sanitize (face->reference_table (HB_OT_TAG_CBDT));

src/hb-ot-hmtx-table.hh

@@ -69,7 +69,7 @@ struct hmtxvmtx
     inline void init (hb_face_t *face,
 		      unsigned int default_advance_ = 0)
     {
-      default_advance = default_advance_ ? default_advance_ : face->get_upem ();
+      default_advance = default_advance_ ? default_advance_ : hb_face_get_upem (face);
 
       bool got_font_extents = false;
       if (T::os2Tag)