calc the output offset if gen moof atom is off
calc the output offset if gen moof atom is off

otherwise the muxing can be inconsistent across different requests

doublex/nginx-vod-module,doublex/nginx-vod-module,doublex/nginx-vod-module,doublex/nginx-vod-module,kaltura/nginx-vod-module,BlackWizard/nginx-vod-module,BlackWizard/nginx-vod-module,kaltura/nginx-vod-module,kaltura/nginx-vod-module,BlackWizard/nginx-vod-module,BlackWizard/nginx-vod-module,kaltura/nginx-vod-module,kaltura/nginx-vod-module,BlackWizard/nginx-vod-module,doublex/nginx-vod-module,BlackWizard/nginx-vod-module,doublex/nginx-vod-module,kaltura/nginx-vod-module