Migrated to clap v3, replacing structopt for handling CLI arguments.

Bug fixes

• The dolby_vision lib now handles variable length metadata blocks correctly.
  • This can be used by L8, L9, and L10 metadata levels.
  • Implements fixes for #104 and #105.
  • Thanks to @saindriches for figuring out most of it.
• When generating a CM v4.0 RPU, a default L9 metadata block is added to conform to Dolby spec.
• It is now possible to inject RPUs into a video file that already contains Dolby Vision RPUs. Fixes #108.
  • In that case, the existing RPUs are ignored and replaced.
• Corrected L3 metadata values from XML.

Features

• RPU generation from XML now supports L10 and L11 metadata. As well as fixes for variable length L8 and L9.
  • L10 metadata is derived from the target displays in the metadata, and can support custom primaries.
• Added a way to edit the existing L9 metadata in RPUs. Only supports preset primaries, refer to documentation.