* Port install_ffmpeg.sh from go-livepeer
* Update ffmpeg and nv-codec-headers versions.
* Use local install_ffmpeg.sh in github CI
* Update transcoder for ffmpeg 7.0.1
* Update tests to be compatible with ffmpeg7 binary
* Fix FPS passthrough
* Set the encoder timebase using AVCodecContext.framerate instead of
the decoder's AVCodecContext.time_base.
The use of AVCodecContext.time_base is deprecated for decoding.
See https://ffmpeg.org/doxygen/3.3/structAVCodecContext.html#ab7bfeb9fa5840aac090e2b0bd0ef7589
* Adjust the packet timebase as necessary for FPS pass through
to match the encoder's expected timebase. For filtergraphs using
FPS adjustment, the filtergraph output timebase will match the
framerate (1 / framerate) and the encoder is configured for the same.
However, for FPS pass through, the filtergraph's output timebase
will match the input timebase (since there is no FPS adjustment)
while the encoder uses the timebase detected from the decoder's
framerate. Since the input timebase does not typically match the FPS
(eg 90khz for mpegts vs 30fps), we need to adjust the packet timestamps
(in container timebase) to the encoder's expected timebase.
* For the specific case of FPS passthrough, preserve the original PTS
as much as possible since we are trying to re-encode existing frames
one-to-one. Use the opaque field for this, since it is already being
populated with the original PTS to detect sentinel packets
during flushing.
Without this, timestamps can be slightly "squashed" down when
rescaling output packets to the muxer's timebase, due to the loss of
precision (eg, demuxer 90khz -> encoder 30hz -> muxer 90khz)
Josh Allmann