[metrics] Implement synthetic slant for caret slope

Part of https://github.com/harfbuzz/harfbuzz/pull/3338

src/hb-ot-metrics.cc

@@ -167,10 +167,18 @@ hb_ot_metrics_get_position (hb_font_t           *font,
     unsigned rise = face->table.hhea->caretSlopeRise;
     unsigned upem = face->get_upem ();
     unsigned mult = rise < upem ? upem / rise : 1;
+
     if (metrics_tag == HB_OT_METRICS_TAG_HORIZONTAL_CARET_RISE)
       return mult * GET_METRIC_Y (hhea, caretSlopeRise);
     else
-      return mult * GET_METRIC_X (hhea, caretSlopeRun);
+    {
+      unsigned run = mult * GET_METRIC_X (hhea, caretSlopeRun);
+
+      if (font->slant)
+	run += _hb_roundf (mult * GET_METRIC_Y (hhea, caretSlopeRise) * font->slant);
+
+      return run;
+    }
   }
   case HB_OT_METRICS_TAG_HORIZONTAL_CARET_OFFSET:     return GET_METRIC_X (hhea, caretOffset);