Bug 927: Fix minor MT issues w/ libav/ffmpeg

Issue:
  [NULL @ 0x35bde60] insufficient thread locking around avcodec_open/close()

Decorating said libav functions w/ mutex lock/release.

Abstract impl. to either use pthread or JNI Monitor,
but using the latter to reduce dependencies (ming64 windows).

FFMPEGNatives is now an abstract class containing the
  'static final Object mutex_avcodec_openclose'