Improve HighDPI support on WinRT

Qt has continuously improved its High DPI support over the year. This is especially true for the Win32 port. WinRT still needs a bit more attention.

Setups containing of a low and a high DPI screen are pretty much broken at the moment. Depending on the initial screen of the application, it might be usable at first but moving it to another screen causes rendering to go haywire. There is also the situation, where the initial painting goes wrong and the application does not recover at all.

The bugs do not only affect setups with multiple screens though. It is also possible that painting goes wrong if only a high DPI screen is attached. Additionally we recently found (and fixed) a bug about wrong mouse coordinates in a high DPI setup.

All in all we have to spend some time to get high DPI support on par with the desktop implementation.