Bug 927 - Multithreading (MT) issues libav/ffmpeg

FFMPEG Natives:

  - Move 'mutex_avcodec_openclose' to local static and initialize at initSymbols0

  - setStream0:
    - Add another locked mutex block around:
       - [ sp_avformat_open_input .. sp_avformat_find_stream_info ]

  This solves the issue of:

    [NULL @ 0x89d20c60] insufficient thread locking around avcodec_open/close()
Bug 927 - Multithreading (MT) issues ALAudioSink

Add global synchronization around ALAudioSink constructor code,
which 'magically' solves the openal-soft multithreading issues ..

This is a workaround.
ALAudioSink: Add thread-name to exceptions; stopImpl shall not throw an exception at destroy
Bug 927 - Multithreading (MT) - GLMediaPlayer.GLMediaEventListener: Remind that commands shall be off-loaded on another thread!