[CDS API] Re-use existing transcoding jobs where possible when performing a time base seek

[Will Lunniss]

When performing a time based seek over the CDS API using &start=pos, Serviio kills the existing transcoding job, and starts a new one at the specified position.

I belive that this is different from how time based seek under the DLNA part of Serviio works, which instead sends already transcoded data to the client if available, else blocks until it is ready.

Could you implement a hybrid of the two such that if the video at the specified position has already been transcoded, send the data from that existing job in the same way it is done for DLNA, and if it isn't ready yet, start a new transcoding job.

[Petr Nejedly]

I have no way of working out the exact second in already transcoded file, I'm using FFmpeg (-s) for that, thus a new transcode job has to be started every time.

Feel free to reopen if you find a solution.