Use wxFindWindowAtPoint() for hit testing in wxPopupTransientWindow.

Use wxFindWindowAtPoint() for hit testing in wxPopupTransientWindow.

This works even with irregularly shaped windows such as wxRichToolTip unlike
the old naive test using wxRect::Contains() which didn't and resulted in us
believing that the mouse was outside the window when it was still in it and
(somehow, the details are not totally clear) recapturing the mouse again and
again in the same window which resulted in assert failures with the new checks
in wxWindowBase::CaptureMouse() or the capture stack corruption before this.

Closes #15288.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74679 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775