Revert wxAuiDefaultDockArt::DrawIcon API change

Mark the function as deprecated. It should not be part of the public API. It
cannot be overridden and manually calling it has no lasting effect because
DrawCaption calls it as well.

Closes https://github.com/wxWidgets/wxWidgets/pull/943

src/aui/dockart.cpp

@@ -632,12 +632,20 @@ void wxAuiDefaultDockArt::DrawCaption(wxDC& dc,
     dc.DestroyClippingRegion();
 }
 
+#if WXWIN_COMPATIBILITY_3_0
+void wxAuiDefaultDockArt::DrawIcon(wxDC& dc, const wxRect& rect, wxAuiPaneInfo& pane)
+{
+    DrawIcon(dc, NULL, rect, pane);
+}
+#endif
+
 void
 wxAuiDefaultDockArt::DrawIcon(wxDC& dc, wxWindow *window, const wxRect& rect, wxAuiPaneInfo& pane)
 {
     // Draw the icon centered vertically
+    int xOffset = window ? window->FromDIP(2) : 2;
     dc.DrawBitmap(pane.icon,
-                  rect.x+window->FromDIP(2), rect.y+(rect.height-pane.icon.GetScaledHeight())/2,
+                  rect.x+xOffset, rect.y+(rect.height-pane.icon.GetScaledHeight())/2,
                   true);
 }