Additional edge-cases/metadata for Will/ArcPNA format

[baconwaifu]

offset 4 in a PNA's frame entry is an 'index' i32, which is what the game itself uses to reference the contained textures; not only do the images often make considerably more sense when sorted by this, I've also encountered images that use -1 as an 'invalid slot' and the rest of the metadata for that frame is null (and there's no associated PNG in the file)

Unfortunately the game I encountered this in is wrapped by Enigma (part of DLSite's DRM) so I can't actually dig into the engine to say what the real thing does with those slots...

[Cosetto]

Did you get the same error?