Add missing detach when clearing font features

Same as when setting the features, we need to detach (but keep
the engine data) when clearing them.

Change-Id: I3adcadfb24c4abfff9c9e1c4f42ed4af919c71f0
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/gui/text/qfont.cpp

@@ -2481,6 +2481,10 @@ bool QFont::isFeatureSet(Tag tag) const
 */
 void QFont::clearFeatures()
 {
+    if (d->features.isEmpty())
+        return;
+
+    d->detachButKeepEngineData(this);
     d->features.clear();
 }