Demonstrates the use of a filter in making GetFeature requests using the WFS protocol.
                If a vector layer has a filter and the protocol supports server-side filtering,
                the filter will be serialized in requests for features.  The WFS protocol can be
                used with a vector layer to serialize a filter using OGC Filter Encoding.  This
                example requests all features that are TYPE "highway" or "road".
            
See the wfs-filter source for details on how this is done.