#7 on-the-fly compress/decompress

[Eugene Bujak]

The demo files are quite big, but they compress very
well.

Now comes the problem that on mass demo recording,
it's quite possible to leave no free space on drives.

Info-Zip's zlib library is a good candidate for being
integrated into opendemo - cpu load isn't high and it
compresses good enough to save around 85% of the
diskspace.

The feature of course should be optional.