COMMON: Reenable SEEK_END seeking in GZipReadStream()

This is needed by the "Mirage" Wintermute game.
Vorbis can do backward seeking, thus we need to enable this for ZIP
streams. Since this can be a potentially slow operation, we throw a
warning (once per stream) when it occurs. Originally, SEEK_END seeks
in GZipReadStream were disabled by commit 9138128f. Refer to patch
#2050337 for more information.