Support IMSC-1 image subtitle profile

[TobbeEdgeware]

There is some interest in image-based subtitles.

The proper way to do that is to follow the W3C IMSC-1 Image Profile, so the suggestion is to follow
http://www.w3.org/TR/ttml-imsc1/#image-profile-constraints.

In particular, the spec says that "A presented image is a div element with a smite:backgroundImage attribute that flows into a presented region."

Our current implementation does have some support for images in the p element, but I suggest that it is removed to discourage non-standard content.

[TobbeEdgeware]

Not allowing self-contained image subtitle media is a nuisance of the IMSC1 spec: it is not enough to download all segments to have the full media in form of video and audio. One must also parse the subtitles segments to find URLs to the subtitle images and download them. For the other media: video and audio, external referencers in the media files are not allowed. Anyway, that is the reason to start with only embedded image subtitles.

[TobbeEdgeware]

Embedded images subtitles is implemented in PR #1536.
I also looked a little at the IMSC-1 case where the actual png images are external URLs. One problem is that the URLs are typically relative URLs and the subtitle engine doesn't know the content URL. @AkamaiDASH, how would you suggest that the content URL should propagate into the TTML parser, or do you see some other solution as preferred?

[TobbeEdgeware]

The right way forward for IMSC-1 compliance seems to be add the images as subsamples after the TTML document in a fragment. That is described in Section 5.6 of ISO 14996-30. It would avoid having external references and would also get away from the base64 encoding of the image data. If someone has reference material for this, it would be great.

[TobbeEdgeware]

This has now been implemented in #1668.