windows hdpi support & opengl

[Boudewijn Rempt]

I'm trying to make Krita work correctly on hi-dpi windows screens 
like the Surface Pro 3 and 4. On OSX with Retina support, I haven't
had complaints, but for Windows there's this bug:

https://bugs.kde.org/show_bug.cgi?id=360541

If I enable

  QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

as is done in

  http://files.kde.org/krita/3/windows/krita3-prealpha3-de0d43d.zip

Users complain that if they enable 150% or 200% display scaling both
the opengl-based and the cpu, qpainter-based canvas are wrong: the
image is also displayed at 150% or 200%.

This is the same image at 100% zoom with scaling:

http://imgur.com/shQ8iyw

when the option is disabled as in

  http://files.kde.org/krita/3/windows/krita-2.99.89.zip

All the ui elements are wrong and sometimes even the svg icons
aren't scaled correctly.

http://imgur.com/VqoSg59

Is there a way I can enable the hidpi scaling for the ui elements,
but handle the opengl and qpainter canvas widget on my own? I'm
looking at http://doc.qt.io/qt-5/highdpi.html which explains why
there's no problem on OSX, with the OpenGL canvas enabled, but not
how I can correctly render images on Windows.

-- 
Boudewijn Rempt | http://www.krita.org, http://www.valdyas.org