Library, Font, and EmitterMold dispose metadata on read, by default.

This frees up memory used by the source .json/.pex/.fnt data, since it's
usually no longer needed after parsing.

This will break projects that create duplicate Library (or
Font/EmitterMold) from the same AssetPack, since the files will be
disposed when it gets to the second load. Pass false to disposeFiles in
that case.