[HiDPI]Coordinate system conversion functions do not work properly with negative coordinates (native child windows)

How to reproduce the issue:

1) You need a system with two screens and the right one has to be the primary
screen, so the left one will have negative screen coordinates. Enable DPI scaling for the left screen in the OS.

2) Run the attached example. (PlaintextEdit in QMainWindow with native winId() enforced).

3) Move the example application window to the left screen. 

4) Open the tooltip. The popup should be somewhere on the wrong screen.