adjust the first key frame offset in get window
adjust the first key frame offset in get window

the first key frame offset is relative to first clip time, so it has to be
updated when the first clip time changes

kaltura/nginx-vod-module,kaltura/nginx-vod-module,kaltura/nginx-vod-module,kaltura/nginx-vod-module,kaltura/nginx-vod-module,kaltura/nginx-vod-module