When starting multiple processes of `mpv --shuffle` in parallel,
sometimes the random seed happens to be identical, so files are
played in the same random order.
mp_rand_seed(0) now uses a random seed provided by libavutil,
and only falls back to time in case of failure.
Fuzzing expect deterministic behavior. While we don't use this random
generator for anything significant, still good to set the seed, in case
of future usage.
Add xoshiro as a PRNG implementation instead of relying
on srand() and rand() from the C standard library. This,
in particular, lets us avoid platform-defined behavior with
respect to threading.
