Fix thread safety issue in MP3Compressor.

[psobot]

Fixes #127.

LAME's hip_decode family of functions makes use of a single static buffer when decoding MP3 frames. This precludes calling hip_decode from multiple threads, as each thread will overwrite the buffer's contents.

This PR fixes the issue by:

• avoiding calling the function that uses the static buffer
• copying the required logic into a lame_overrides.c file, which implements hip_decode_threadsafe
• calling hip_decode_threadsafe from MP3Compressor instead.
• adding a test to ensure that all deterministic plugins generate identical output when called from multiple threads concurrently

(cc @iCorv)