a11y AT-SPI: incorrect handling of window-relative coords for "GetOffsetAtPoint" (text interface)

Window-relative coordinates passed to the "GetOffsetAtPoint" method of the AT-SPI text interface are handled incorrectly.

Note: This is even with the following pending fixes for other issues in place (as was the case when testing this, since there are even more issues to run into otherwise):

https://codereview.qt-project.org/c/qt/qtbase/+/422251

https://codereview.qt-project.org/c/qt/qtbase/+/422333

Steps to reproduce (s.a. attached screencast):

1) start Accerciser (https://wiki.gnome.org/Apps/Accerciser)

2) build and run the "textedit" example from qtbase:
$ qtbase/examples/widgets/richtext/textedit/textedit

3) navigate to the example's text node in Accerciser's a11y treeview of the application
4) move the window to the top-left of the (first) screen
5) type the following into Accerciser's IPython console to check the result of calling the "GetOffsetAtPoint" AT-SPI method on the selected item. In the screencast::

In [10]: acc.queryText().getOffsetAtPoint(300, 300, pyatspi.component.XY_WINDOW)

Out [11]: 835

6) move the window somewhere else

7) repeat step 5

Actual result:

A different value than in step 5 is returned. Here:

In [11]: acc.queryText().getOffsetAtPoint(300, 300, pyatspi.component.XY_WINDOW)

Out [12]: -1

Expected result:

Since window-relative coordinates have been specified (__pyatspi.component.XY_WINDOW), the result should be independent of the screen position of the window, i.e. the same result should be returned in steps 5) and 7). Only screen coordinates should change as the window is moved around, windows-relative coordinates shouldn't.