fcp_xml: serialize marker out value instead of -1#1411

ssteinbach merged 1 commit intoAcademySoftwareFoundation:mainAcademySoftwareFoundation/OpenTimelineIO:mainfrom

jhorrocks-tl:fcpxml_support_marker_outjhorrocks-tl/OpenTimelineIO:fcpxml_support_marker_outCopy head branch name to clipboard