The cursor didn't change to a link one when the mouse was over link areas in
an image map.
Fix this by generalizing wxHtmlCell::GetMouseCursor() into GetMouseCursorAt().
Closes#15350.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74570 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775