This tries to handle cases where separate invocations of decode_frame()
(each running in separate threads) write to respective fields in the
same AVFrame->data[]. Having per-field owners makes interaction between
readers (the referencing thread) and writers (the decoding thread)
slightly more optimal if both accesses are field-based, since they will
use the respective producer's thread objects (mutex/cond) instead of
sharing the thread objects of the first field's producer.

In practice, this fixes the following tsan-warning in fate-h264:

WARNING: ThreadSanitizer: data race (pid=21615)
  Read of size 4 at 0x7d640000d9fc by thread T2 (mutexes: write M1006):
    #0 ff_thread_report_progress pthread_frame.c:569 (ffmpeg:x86_64+0x100f7cf54)
[..]
  Previous write of size 4 at 0x7d640000d9fc by main thread (mutexes: write M1004):
    #0 update_context_from_user pthread_frame.c:335 (ffmpeg:x86_64+0x100f81abb)