Note that this only supports a single page within the DVB-sub stream.
Multiple pages will be merged together. A follow-up will allow
selecting a specific page.
This only supports outputting using TTML or MP4+TTML; you cannot have
DVB-sub output nor can you output it in WebVTT. Since DVB-sub
uses images, it is hard to impossible to do this with WebVTT.
This also only supports interlaced images, not progressive images
nor text.
Closes#832
Change-Id: Id6dbb6393c7b9a05722e61c6bd255bef5e69a7d8
78be14c092