canvas: signal that DASH or HLS has captions

[jronallo]

Manifests for DASH and HLS can include captions and subtitles. For instance HLS can have CEA-708 captions within the MPEG-2 video stream. A WebVTT file can also be pointed to from a DASH or HLS manifest. If there are sidecar captions (say WebVTT) in a seeAlso you won't need to use those sidecar captions if they're already included in the DASH or HLS manifest and you are using a video player than can use these captions. Including both the DASH/HLS manifest captions and the seeAlso captions to a player can cause problems or doubling up in controls.

Solution could be to have a way to show that the DASH or HLS stream includes captions. Or the shim to a player could first check whether the stream has captions and if not use the seeAlso captions.