This patch fixes strange seeking behavior i have observed in mpv when using the libopenmpt demuxer, caused by mismatch in position state between the demuxer and underlying libopenmpt library.  Not setting the presentation timestamp field of the AVPacket caused it to be guessed by libavformat, but the guess didn't take seeking into account, so after seeking libopenmpt produced packets at the new seeked position but they were then presented as if they belong to the old position!  A quick check for this behavior is to open a tracker tracker module that is, for example 2 minutes and a few seconds long in mpv, and then press "up" 2 times (which would seek 2 minutes forward): buggy demuxer causes mpv to exit immediately after seek because of an EOF,  with non-buggy demuxer mpv will play the last few seconds of the song and then exit.

I found the bug when writing my own demuxer for an obscure audio format (which i'm not quite sure if i should submit too) as i was using the libopenmpt one as a reference and copied the bug too.  Testing was done on 6.1.1, but libopenmpt demuxer's code didn't really change since then so it should be relevant for the latest one too (unless the default value for pkt->pts changed to take seeking into account), and either way it makes more sense to expose information that is very much available to the demuxer rather than leave it to be guessed.