SRB2WADDIR doesn't work with relative paths

In IdentifyVersion, srb2 calls I_LocateWad() to get the location of the wad directory, then appends "srb2.pk3" to the path and opens the file. However, I_LocateWad() itself changes to the WAD directory.

This means that relative paths in SRB2WADDIR don't work; after we change directories, the path is no longer valid. Requiring the use of absolute paths reduces the usefulness of the environment variable. For example, one case where it would otherwise be useful is having a portable installation with startup script that points srb2 to the correct wad directory relative to the executable. Sure, srb2 looks in the same directory as the executable, but only that directory, not ./wads, or ./usr/share/srb2 (useful for AppImage).

In the case where srb2 changes directories, shouldn't the path to the WAD be just "srb2.pk3" and not "some/path/srb2.pk3"? Or alternatively, is it necessary to change directories to the wad dir? I realize the code here has a lot of paths going through different systems, but it seems like it should be possible to make this work.