MSVC: Disable strict Qt iterators for Qt 5.13+

MSVC is unable to disambiguate between definitions of QVector<QPointF>
and QPolygonF. This is a known upstream bug, e.g.:

- https://phabricator.kde.org/D21314
- https://codereview.qt-project.org/c/qt/qtbase/+/180229



Starting with Qt 5.13, it is impossible to use strict iterators
wholesale because of the following commit:

commit 972f8845a85d6a07140025e4257cb8a1a2699b5d
Author: Lars Knoll <lars.knoll@qt.io>
Date:   Mon Oct 8 11:55:13 2018 +0200

    Invert include dependencies between QList and QVector

    This is a very slight source incompatibility, but required as
    a preparation for Qt 6, where QList should inherit QVector or
    share the implementation with it.

    This requires some special work to correctly instantiate and
    export QVector<QPoint> from Qt Core on MSVC.

    Change-Id: I1d042c5fafdde7afe59409eda2580871d4832fcd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

This emergency change is necessary because otherwise the build won't
even get past kritaglobal, which uses that one specific class.

CCMAIL: kimageshop@kde.org