Fix animation cache not working when image is behind a symlink

[mendess]

I noticed that for large gifs swww takes a long time to send them to the daemon. I tried seeing if I could parallelize the process somehow but instead found that the cache just wasn't working. Fixing the cache gives a lot more performance than parallelizing the compression of the frames.

Parallelizing the compression can however be done. I did a little bit of investigating and realized that, while pack_bytes has to be done sequentially. Compressing lz4 itself doesn't need to be. And after measuring the time each of those two operations take, it might be worth it.

Rough measurments in a busy machine

pack_bytes_timings: Timings { 
    min: Some(1.477043ms),
    avg: 5.156188ms,
    max: 11.548624ms,
    total: 1.057301553s,
    count: 239
}
lz4_compress_timings: Timings {
    min: Some(232.441437ms),
    avg: 290.059601ms,
    max: 321.532978ms,
    total: 68.270607189s,
    count: 239
}

[LGFae]

Parallelizing the compression can however be done.

I investigated this some time ago and decided against it since the cache already solves this problem, and I thought that parallelizing it would make it too complicated. Maybe we could try it again...

In any case, thanks for the patch!