Change wxPGProperty::OnCustomPaint measure item call signature

Because horizontally scrolled wxPGProperty can have x-coordinate < 0 so there is a need to change measure item call signature from rect.x < 0 condition to something more specific to avoid misinterpretation of the calls.

src/propgrid/editors.cpp

@@ -797,7 +797,7 @@ void wxPropertyGrid::OnComboItemPaint( const wxPGComboBox* pCb,
         cis = GetImageSize(p, item);
     }
 
-    if ( rect.x < 0 )
+    if ( rect.x + rect.width < 0 )
     {
         // Default measure behaviour (no flexible, custom paint image only)
         if ( rect.width < 0 )