Restore fullscreen on Windows
Summary:
Menus on top of fullscreen OpenGL windows were not visible.
There's a bunch of finger pointing with Qt saying Windows' OpenGL
implementation is broken. A workaround is added in Qt 5.6.
http://doc.qt.io/qt-5/windows-issues.html
Fullscreen on Windows will then add a 1px border around the window by
default. This can be worked around by patching Qt.
https://bugreports.qt.io/browse/QTBUG-41309
Reviewers: Krita:_next
Differential Revision: https://phabricator.kde.org/D1977