The VP9 level is computed when the container is missing a codec config
or if the level is missing from the codec config.
This fixes VP9 in ISO-BMFF files generated by FFmpeg v4.0.2 or earlier
which does not have level set in the codec config.
Fixes#469.
Change-Id: I685bfd48be16ee6b2209da1c3173f7d6bb02b36a