[Bug] Compression uses legacy mode only

[Xenira]

Old versions of pak files (<8) use an index to identify the compression method:
0: None
1: Zlib
2: Gzip
3: Custom

Pak files >= 8 use name based compression index (in footer). Using the current implementation the index used might not match with the corresponding legacy compression method.

As currently only zlib is supported this should not cause problems, but should be fixed when implementing more methods and as validation to make sure the used method is supported.