Fall back on ::GetMessagePos() if ::GetCursorPos() fails under MSW.

In some rare but reproducible cases GetCursorPos() can fail and return without
filling in the provided point. Fall back to GetMessagePos() if this happens:
this is not ideal but clearly better than using uninitialized position or hard
coding something like (0, 0).

Closes #13664.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69758 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/private.h

@@ -923,6 +923,9 @@ WXDLLIMPEXP_BASE wxWinVersion wxGetWinVersion();
 extern HCURSOR wxGetCurrentBusyCursor();    // from msw/utils.cpp
 extern const wxCursor *wxGetGlobalCursor(); // from msw/cursor.cpp
 
+// GetCursorPos can fail without populating the POINT. This falls back to GetMessagePos.
+WXDLLIMPEXP_CORE void wxGetCursorPosMSW(POINT* pt);
+
 WXDLLIMPEXP_CORE void wxGetCharSize(WXHWND wnd, int *x, int *y, const wxFont& the_font);
 WXDLLIMPEXP_CORE void wxFillLogFont(LOGFONT *logFont, const wxFont *font);
 WXDLLIMPEXP_CORE wxFont wxCreateFontFromLogFont(const LOGFONT *logFont);