playloop: make --loop aware of options that modify the start time

[llyyr]

Currently we just seek to the first frame marked by the demuxer, which is wrong because the user might use the --start option together with --loop.

[na-na-hi]

I don't think this is a bug. The documentation says:

Seek to given time position.

I expect when the file is loaded, seek to that position, but otherwise it should have no lingering effects.

If you want the behavior described here (basically rebasing the file start to the specified position) it should be a separate option.

[llyyr]

I don't think this is a bug.

You're right, I didn't look at what the documentation for --start actually said but I don't think it's supposed to have any lingering effects after use. I'll just close this then because this is an extremely niche use case and not useful for anything when loop-playlist exists anyway