Don't redraw wxSplitterWindow sash needlessly

Don't draw the sash in SizeWindows() unless its position was changed by
the function.

src/generic/splitter.cpp

@@ -662,6 +662,8 @@ void wxSplitterWindow::SetSashPositionAndNotify(int sashPos)
 // including the edges next to the sash.
 void wxSplitterWindow::SizeWindows()
 {
+    int oldSashPosition = m_sashPosition;
+
     // check if we have delayed setting the real sash position
     if ( m_requestedSashPosition != INT_MAX )
     {
@@ -727,8 +729,11 @@ void wxSplitterWindow::SizeWindows()
         GetWindow1()->SetSize(border, border, w1, h1);
     }
 
-    wxClientDC dc(this);
-    DrawSash(dc);
+    if ( oldSashPosition != m_sashPosition )
+    {
+        wxClientDC dc(this);
+        DrawSash(dc);
+    }
 }
 
 // Set pane for unsplit window