Shapefile renderer cascade onto StreamingRenderer triggers memory leaks

When cascading to StreamingRenderer a new MapContext is built, which attaches a new set of listeners to the feature sources. These get never cleared, resulting in a progressive memory leak (FeatureListenerManager keeps around the new mapcontext with all its style information as well as the feature source, if the style is big the out of memory happens relatively soon, if it's small it takes lots of time).