Fix QGtkPainter::reset() to reset the clip rect

Due to preparation for GTK3 support, QGtkStyle::drawXxx() no longer
creates a local instance of QGtkPainter upon every call, but
QGtkStylePrivate::gtkPainter() will (in the future) dynamically choose
between QGtk[2|3]Painter. The same painter instance is now re-used
between the calls, but wasn't properly reseted between.

Task-number: QTBUG-28557
Change-Id: I29be318dbb54d97ed23dfb88d406a3a833db4369
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/widgets/styles/qgtkpainter.cpp

@@ -63,6 +63,7 @@ void QGtkPainter::reset(QPainter *painter)
     m_hflipped = false;
     m_vflipped = false;
     m_usePixmapCache = true;
+    m_cliprect = QRect();
 }
 
 QString QGtkPainter::uniqueName(const QString &key, GtkStateType state, GtkShadowType shadow,