To fix a data race, commit 5caeab5f6d
called video_buffer_push() and video_buffer_consume() under the
v4l2_sink lock.
Instead, use the previous_skipped indication (initialized with video
buffer locked) to lock only for protecting the has_frame flag.
This enables the possibility for the video_buffer to notify new frames
via callbacks without lock inversion issues.
The v4l2_sink implementation directly read the internal video_buffer
field "pending_frame_consumed", which is protected by the internal
video_buffer mutex. But this mutex was not locked, so reads were racy.
Lock using the v4l2_sink mutex in addition, and use a separate field to
avoid depending on the video_buffer internal data.
From FFmpeg/doc/APIchanges:
2021-03-17 - f7db77bd87 - lavc 58.133.100 - codec.h
Deprecated av_init_packet(). Once removed, sizeof(AVPacket) will
no longer be a part of the public ABI.
Refs #2302 <https://github.com/Genymobile/scrcpy/issues/2302>
