WinRT: hidpi scaling issues.

The hidpi changes introduced in https://codereview.qt-project.org/#/c/142093/ works great if your display has a 1.0 or 2.0 pixel density. However for some displays this does not work as expected.

For example if the logical dpi for the display is 240, the calculated pixel density is rounded up from 2.5 to 3.0. What this means is that the UI components are over scaled and look unnatural for the size of the screen i.e. 0.5x larger than they should be.

If you remove the qRound function then the UI is scaled correctly for the display.