WIP: CTA608(EIA608) caption PoC

[vincentsong]

This is a PoC of parsing and displaying CTA-608(EIA608) closed caption

• [x] Extracting CC from h264 SEI user data registered
• [x] Parsing and decoding the CC
• [x] Rendering the CC on HTML video element
• [x] Improve the callback of generating the new cue CC in CTA608 parser in parser cache
• [ ] Move the caption caching and searching logic into SDK
• [ ] Return required caption attributes such as styles and layout

Note: we are using requestVideoFrameCallback in demo app to render the closed caption, but this API is not available in FireFox so far, so we need to find an alternative solution on firefox.

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Latest commit: 7115561189b1e432dc626031cdbd06256ce06e29

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