A useful vanilla-compatible trick is that it's possible to include
sprites as patches inside texture definitions. It turns out that
both Final Doom IWADs added textures which do this, and Freedoom
inherits this as a result. We shouldn't include these sprites in the
pnames.txt that gets generated for inclusion in wadinfo.txt, since
they're already defined in wadinfo.txt under [sprites] and won't
be found under patches/ anyway.
This should help as a step towards resolving #485.
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