Refactor song decoder

[madmaxoft]

Refactor the song decoder code to return the fully decoded buffer, rather than "streaming into" a buffer.

At the same time, check why Dj Maksy & Sia - My Love decodes only the first minute, rather than the entire song.

[madmaxoft]

Closing - the decoding takes two seconds on a fast computer, would be too slow for slow computers.

Decoding partial songs fixed - ignoring the errors from avcodec_send_packet.