Add wxAnyScrollHelperBase to reduce code duplication in wxVarScrollHelperBase.

This is just a small refactoring to move some trivially common parts of
wxScrollHelperBase and wxVarScrollHelperBase in a new common base class.
This will make it possible to apply other corrections to wxVarScrollHelperBase
without having to physically duplicate the code from wxScrollHelperBase in it.

See #15357.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74813 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/vscroll.cpp

@@ -139,9 +139,8 @@ bool wxVarScrollHelperEvtHandler::ProcessEvent(wxEvent& event)
 // ----------------------------------------------------------------------------
 
 wxVarScrollHelperBase::wxVarScrollHelperBase(wxWindow *win)
+    : wxAnyScrollHelperBase(win)
 {
-    wxASSERT_MSG( win, wxT("associated window can't be NULL in wxVarScrollHelperBase") );
-
 #if wxUSE_MOUSEWHEEL
     m_sumWheelRotation = 0;
 #endif
@@ -150,17 +149,11 @@ wxVarScrollHelperBase::wxVarScrollHelperBase(wxWindow *win)
     m_sizeTotal = 0;
     m_unitFirst = 0;
 
-    m_win =
-    m_targetWindow = NULL;
-
     m_physicalScrolling = true;
     m_handler = NULL;
 
-    m_win = win;
-
     // by default, the associated window is also the target window
     DoSetTargetWindow(win);
-
 }
 
 wxVarScrollHelperBase::~wxVarScrollHelperBase()