The first frames are typically received and decoded with more delay than
the others, causing a wrong slope estimation on start.
To compensate, assume an initial slope of 1, then progressively use the
estimated slope.
To minimize latency (at the cost of jitter), scrcpy always displays a
frame as soon as it available, without waiting.
However, when recording (--record), it still writes the captured
timestamps to the output file, so that the recorded file can be played
correctly without jitter.
Some real-time use cases might benefit from adding a small latency to
compensate for jitter too. For example, few tens of seconds of latency
for live-streaming are not important, but jitter is noticeable.
Therefore, implement a buffering mechanism (disabled by default) to add
a configurable latency delay.
PR #2417 <https://github.com/Genymobile/scrcpy/issues/2417>
Currently, a frame is available to the consumer as soon as it is pushed
by the producer (which can detect if the previous frame is skipped).
Notify the new frames (and frame skipped) via callbacks instead.
This paves the way to add (optional) buffering, which will introduce a
delay between the time when the frame is produced and the time it is
available to be consumed.
The current video buffer only stores one pending frame.
In order to add a new buffering feature, move this part to a separate
"frame buffer". Keep the video_buffer, which currently delegates all its
calls to the frame_buffer.
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 function sc_cond_timedwait() accepted a parameter representing the
max duration to wait, because it internally uses SDL_CondWaitTimeout().
Instead, accept a deadline, to be consistent with
pthread_cond_timedwait().
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