[WIP] User api

[lu-zero]

See issue #410

API:

• Config struct with all the configuration items
  • FromStr to parse a x264-like string
  • Display to produce a x264-like string
  • parse_option(o: &str) to update the configuration using a x264-like key:value string
  • new_context() -> Context
• Context: opaque struct embedding the existing FrameInvariants, Sequence
  • new_frame() -> Arc<Frame>
  • receive_obu_header() -> Vec<u8>
  • send_frame(frame: Arc<Frame>)/receive_packet() -> Packet
  • flush support
• Packet
  • data buffer
  • user private data
  • timestamp

Usage:

• rav1e/rav1repl
• decode_test

Refactor:

• Arc the input frame
• Arc the reconstructed frame

[tdaede]

This looks okay, but can you explain the rationale for Arc? Is it explained somewhere else?

[lu-zero]

It is useful for the multithread experiments (the rec part needs the same changes, but @fbossen is already working on it) and to have multi-encode (see what the commercial x265 offers).

[tdaede]

I guess I buy it. I kind of imagined rav1e eating the frames once you pass them in, but I guess there is no reason to.

[lu-zero]

Rebased to the current master. Once the CI confirms it is working I'd merge it and prepare a new set for the remaining items.