To fix a data race, commit 5caeab5f6d12fe0fc69e708a829cfdbec3a401b4
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.
28bce48d47