Clear full sprite area for fdo#45219

Use clearAll() here, to really clear the whole sprite area (clear()
does respect potential clip polygons)

slideshow/source/engine/slide/layer.cxx

@@ -222,7 +222,7 @@ namespace slideshow
             std::for_each( maViewEntries.begin(),
                            maViewEntries.end(),
                            boost::bind(
-                               &ViewLayer::clear,
+                               &ViewLayer::clearAll,
                                boost::bind(
                                    &ViewEntry::getViewLayer,
                                    _1)));