[dvdplayer] calculate seekoffset when performing chapter seeks

This calculates the missing seekoffset value when performing chapter seeks and pushes it the infomanager. Time/percentage bases seeks already do this (kind of). I am not entirely sure if it's done right. There might be a better way. @FernetMenta for review please.

Bug report @ http://forum.kodi.tv/showthread.php?tid=210778

Ha! Makes sense :-) Thanks much. No sanity checks needed if start is NOPTS? (e.g. if we don't find a chapter?)