Fix wxScrollHelperEvtHandler broken by recent changes to event processing.

Use ProcessEventLocally() added in r64261 (which was probably the one to break
this) to forward event to the window itself instead of ProcessEvent() in
wxScrollHelperEvtHandler::ProcessEvent() implementation. Calling ProcessEvent()
didn't work any more in a case when another event handler was pushed on a
wxScrolledWindow: in this case the EVT_SIZE and EVT_PAINT handlers defined in
the window itself were not called at all any more.

Add a unit test checking for the even more tortuous than usual event
processing path in this particular case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64358 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/scrlwing.cpp

@@ -204,7 +204,7 @@ bool wxScrollHelperEvtHandler::ProcessEvent(wxEvent& event)
     m_hasDrawnWindow = true;
 
     // pass it on to the real handler
-    bool processed = wxEvtHandler::ProcessEvent(event);
+    bool processed = m_nextHandler->ProcessEventLocally(event);
 
     // always process the size events ourselves, even if the user code handles
     // them as well, as we need to AdjustScrollbars()