Closes #453. Change-Id: I68e46fb83cbf7e62a19fa83698cb66bfc0acd98d
Implemented according to bitstream spec at https://aomediacodec.github.io/av1-spec/. It will be used to do AV1 subsample encryption later. Issue #453. Change-Id: I84d8a2a780d95f2c9f430ee598838b97474cc0af