Inserting Slate Does Not Step Subtitle Media Sequence Number Correctly

[Nfrederiksen]

When the CE fails to load the scheduled vod, it is expected to insert a slate instead as a fallback.
It has been discovered that when running CE on demux mode with subtitles, and this case occurs something strange happens.
The slate gets loaded and inserted as planned, but the EXT-X-MEDIA-SEQUENCE count on the subtitle tracks decreases, ie going backward. This is unattended behavior and should be fixed.