Forced subtitle DASH support checking for wrong Role@value

[theRealRobG]

When checking for forced subtitle behaviour in the DashManifestParser.java, the Role@value being matched against is forced_subtitle:
https://github.com/google/ExoPlayer/blob/r2.16.1/library/dash/src/main/java/com/google/android/exoplayer2/source/dash/manifest/DashManifestParser.java#L1477
https://github.com/google/ExoPlayer/blob/r2.16.1/library/dash/src/main/java/com/google/android/exoplayer2/source/dash/manifest/DashManifestParser.java#L1547

But according to the 4th edition of ISO/IEC 23009-1 (publicly available here) this should be checking for forced-subtitle. This is defined in Table 31 — Role@value attribute for scheme with a value "urn:mpeg:dash:role:2011":

Role@value	Description	Applicable Media Type(s) (Informative)
forced-subtitle	Textual information meant for display when no other text representation is selected. It is used to clarify dialogue, alternate languages, texted graphics or location/person IDs that are not otherwise covered in the dubbed/localized audio.	text

[icbaker]

Thanks for pointing this out - as you've probably realised, we added support for this after it was requested in #8781 and we used forced_subtitled based on this comment from April 2018: Dash-Industry-Forum/DASH-IF-IOP#165 (comment)

Given you've mentioned later in that thread that Shaka have have also done the same thing, I suspect there will now be content out there with a mix of the two spellings :( It's probably best for now if I update ExoPlayer to support both spellings - so that content that used to work with ExoPlayer (with the 'wrong' spelling) will keep working. I'll make that change.

[theRealRobG]

Yes, makes sense as a pragmatic approach.

Thanks for quick reply!