ZIP/PK3 support
ZDoom allows the use of ZIP files to hold map data instead of the WAD format. It allows for directories as well as filenames that are longer than 8 characters, but ignores directory ordering (or at the very least, lumps in these should be order-agnostic). ZDoom adheres to a specific directory structure: http://zdoom.org/wiki/Using_ZIPs_as_WAD_replacement
We now use zlib in the codebase (for libpng so far).