Subject: [Cellar] Misc Encryption updates

From: Steve Lhomme <[email-masked]>
Date: Thu, 11 Oct 2018 12:53:41 +0200

Following my work on the AV1 encryption I refreshed the encryption 
element text/formatting, added some extra text.

Some new pull requests in case anyone has issues with the added texts:

- ContentEncryption was supposed to be absent in some cases but we can't 
require that. We can require 
<https://github.com/Matroska-Org/matroska-specification/pull/279>that 
it's ignored
https://github.com/Matroska-Org/matroska-specification/pull/279

- the frame encryption wasn't clear what part of a Block it would 
encrypt/compress. It should be each frame, excluding the lacing parts
https://github.com/Matroska-Org/matroska-specification/pull/280

- make ContentEncAlgo mandatory, it has a default value of no encryption 
anyway
https://github.com/Matroska-Org/matroska-specification/pull/281

I also added the 2 elements found in WebM that were not in Matroska:
https://github.com/Matroska-Org/matroska-specification/pull/282

Too bad AESSettingsCipherMode doesn't have a default value, otherwise 
picking the AES encryption would automatically pick the only mode 
currently allowed in WebM.