a11y AT-SPI: Window-relative positions are incorrect

When requesting window-relative positions via AT-SPI, those return a wrong value for the y coordinate.

Sample steps to reproduce using the qt6 variant of LibreOffice (s.a. attached screencast):

1) open a document containing a few paragraphs in LibreOffice Writer using the qt6 variant (not included in official from The Document Foundation yet, can be enabled using the `--enable-qt6` build switch)

2) start Accerciser

3) navigate to the first paragraph in Accerciser's tree view

4) print the screen coordinates and the window-relative coordinates using Accerciser's IPython console, e.g.:

In [18]: acc.queryComponent().getPosition(pyatspi.component.XY_SCREEN)
Out[19]: (1972, 245)
In [19]: acc.queryComponent().getPosition(pyatspi.component.XY_WINDOW)
Out[20]: (52, 23)

5) navigate to the next paragraph in the treeview, print its coordinates:

In [20]: acc.queryComponent().getPosition(pyatspi.component.XY_SCREEN)
Out[21]: (1972, 263)
In [21]: acc.queryComponent().getPosition(pyatspi.component.XY_WINDOW)
Out[22]: (52, 23)

Expected behavior: As compared to the position of the previous paragraph, the y coordinate for the window-relative position (pyatspi.component.XY_WINDOW) should have increased by the same value as the screen coordinate.

Actual behavior: The y coordinate of the screen-relative position is incorrect and remains unchanged (s. "Out[20]" and "Out[22]" above).