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

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>
(cherry picked from commit 0b0b74a563d1513e7857000526d5c0fa462ff6af)
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>