Don’t prefilter rings.

Prefiltering ignores the topology, which means it effectively ignores --filter
=small-detached, removing rings that are smaller than the simplification
threshold but that are attached to other rings (such as census tracts in
Manhattan).

Also, prefiltering interferes with automatic scaling using --width and --height
because it removes rings before the transform has been scaled! Thus, if you try
to use --simplify and --projection without specifying a scale, instead scaling
with --width, prefilter could remove many more rings than intended.