WIP: XCB: Support high-dpi scaling

The QWindow API can now return values in device
independent pixels when QT_HIGHDPI_SCALE_FACTOR is
set.

The platform plugins operate in device pixels. Add
conversion calls for values obtained from QWindow.
(these are no-ops when QT_HIGHDPI_SCALE_FACTOR is
not set)

Task-number: QTBUG-38858
Change-Id: Id577d6413ab0bd2da67a64fe7a643a310e50d72e