The method qApp->devicePixelRatio() should only be used under
special circumstances.  The documentation of QGuiApplication says:

  "Returns the highest screen device pixel ratio found on the system.
   [...] Use this function only when you don't know which window
   you are targeting. If you do know the target window,
   use QWindow::devicePixelRatio() instead."

The Okular code used qApp->devicePixelRatio() a few times without
reason.  Few people noticed this, because apparently only Windows
and Wayland can set per-screen dprs.  This patch replaces those
calls with the correct QWindow call.