timeshift: fix ff,rew,skip+,skip-

For me, this changes fixes all timeshift issues with ff,rew,skip+,skip- (the well-known "allways skips to start of buffer bug").

Not quite sure why not subtracting pkt_pts from "now" fixes things, but it does. Any opinions/explanation?

Are the values of pkt->pkt_pts as returned now the real culprit? Don't know.