avformat/movenc: write the colr atom by default

avformat/movenc: write the colr atom by default

The write_colr flag has been marked as experimental for over 5 years.
It should be safe to enable its behavior by default as follows:

  - Write the colr atom by default for mp4/mov if any of the following:
     - The primaries/trc/matrix are all specified, OR
     - There is an ICC profile, OR
     - The user specified +write_colr
  - Keep the write_colr flag for situations where the user wants to
    write the colr atom even if the color info is unspecified (e.g.,
    http://ffmpeg.org/pipermail/ffmpeg-devel/2020-March/259334.html)

This fixes https://trac.ffmpeg.org/ticket/7961

Signed-off-by: Michael Bradshaw <mjbshaw@google.com>