Add support for Dolby Vision profile 20

[quietvoid]

Samples: https://developer.apple.com/streaming/examples/

[Rainbaby]

It seems like verifier cli (v4.5.0) doesn't support it yet.

[quietvoid]

Yes it is mentioned in the Dolby page.

Dolby Vision Verification Toolkit: Currently Available upon request

[quietvoid]

The only difference seems to be that el_bit_depth_minus8=58370 is failing validation but if ignoring the rest parses fine.

[Nomis101]

There is now support for el_bit_depth_minus8 and ext_mapping_idc in FFmpeg.
FFmpeg/FFmpeg@0c0e7ec

[quietvoid]

I guess I'll merge the PR with the FFmpeg naming.