Move min/max size constrain logic to QPlatformWindow.

This increases the uniformity of the to/fromNativePixels
functions by removing a special case, and avoids pulling
the qwindow_p.h details into qhihgdpiscaling.cpp.

Keep the logic of constraining the device-independent
size.

Change-Id: Id0dae30a7c0170ff2e5d21b94d56719cc900c726
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>