QFontVariableAxis: add debug stream support

Was missing from this new value type, and is quite useful for this type.

Addresses API review comment.

Change-Id: I07ea7ded7f963b2032c377f024bb88e9d54a4d5c
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit b562c36a9a7be5f62a8e79b93c9728381775f460)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/text/qfontvariableaxis.cpp

@@ -3,6 +3,8 @@
 
 #include "qfontvariableaxis.h"
 
+#include <QtCore/qdebug.h>
+
 QT_BEGIN_NAMESPACE
 
 class QFontVariableAxisPrivate : public QSharedData
@@ -223,4 +225,24 @@ void QFontVariableAxis::detach()
     d_ptr.detach();
 }
 
+#ifndef QT_NO_DEBUG_STREAM
+QDebug operator<<(QDebug debug, const QFontVariableAxis &axis)
+{
+    QDebugStateSaver save(debug);
+
+    debug.nospace().noquote();
+    const QString name = axis.name();
+    if (!name.isEmpty())
+        debug << name << '(';
+
+    debug << axis.tag();
+    if (!name.isEmpty())
+        debug << ')';
+    debug << '[' << axis.minimumValue() << "..." << axis.maximumValue()
+          << "; default=" << axis.defaultValue() << ']';
+
+    return debug;
+}
+#endif
+
 QT_END_NAMESPACE

src/gui/text/qfontvariableaxis.h

@@ -56,6 +56,10 @@ private:
     QByteArray tagString() const { return tag().toString(); }
     void detach();
 
+#ifndef QT_NO_DEBUG_STREAM
+    Q_GUI_EXPORT friend QDebug operator<<(QDebug debug, const QFontVariableAxis &axis);
+#endif
+
     QExplicitlySharedDataPointer<QFontVariableAxisPrivate> d_ptr;
 };