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.