[metrics] Fix weird use of xor

I believe that was a try to use one approach as fallback to other.  But
felt wrong.  Just believe what's in OS/2 table to be correct.

src/hb-ot-metrics-internal.cc

@@ -55,14 +55,14 @@ hb_ot_metrics_get_position_internal (hb_face_t       *face,
   (face->table.TABLE->has_data () && \
     (position && (*position = face->table.TABLE->ATTR + face->table.MVAR->get_var (metrics_tag, nullptr, 0)), true))
   case HB_OT_METRICS_HORIZONTAL_ASCENDER:
-    return (face->table.OS2->is_typo_metrics () ^ GET_METRIC (hhea, ascender)) ||
-	   GET_METRIC (OS2, sTypoAscender);
+    return face->table.OS2->use_typo_metrics () ?
+	   GET_METRIC (OS2, sTypoAscender) : GET_METRIC (hhea, ascender);
   case HB_OT_METRICS_HORIZONTAL_DESCENDER:
-    return (face->table.OS2->is_typo_metrics () ^ GET_METRIC (hhea, descender)) ||
-	   GET_METRIC (OS2, sTypoDescender);
+    return face->table.OS2->use_typo_metrics () ?
+	   GET_METRIC (OS2, sTypoDescender) : GET_METRIC (hhea, descender);
   case HB_OT_METRICS_HORIZONTAL_LINE_GAP:
-    return (face->table.OS2->is_typo_metrics () ^ GET_METRIC (hhea, lineGap)) ||
-	   GET_METRIC (OS2, sTypoLineGap);
+    return face->table.OS2->use_typo_metrics () ?
+	   GET_METRIC (OS2, sTypoLineGap) : GET_METRIC (hhea, lineGap);
   case HB_OT_METRICS_HORIZONTAL_CLIPPING_ASCENT:  return GET_METRIC (OS2, usWinAscent);
   case HB_OT_METRICS_HORIZONTAL_CLIPPING_DESCENT: return GET_METRIC (OS2, usWinDescent);
   case HB_OT_METRICS_VERTICAL_ASCENDER:           return GET_METRIC (vhea, ascender);

src/hb-ot-os2-table.hh

@@ -119,7 +119,7 @@ struct OS2
 
   bool is_italic () const       { return fsSelection & ITALIC; }
   bool is_oblique () const      { return fsSelection & OBLIQUE; }
-  bool is_typo_metrics () const { return fsSelection & USE_TYPO_METRICS; }
+  bool use_typo_metrics () const { return fsSelection & USE_TYPO_METRICS; }
 
   enum width_class_t {
     FWIDTH_ULTRA_CONDENSED	= 1, /* 50% */