This commit:
1. Reads the WebM and Matroska ContentEncryption subelements.
2. Creates a GST_PROTECTION event for each ContentEncryption, which
will be sent before pushing the first source buffer.
The DRM system id field in this event is set to GST_PROTECTION_UNSPECIFIED_SYSTEM_ID,
because it isn't specified neither by Matroska nor by the WebM spec.
3. Reads the protection information of encrypted Block/SimpleBlock and
extracts the IV and the partitioning format (subsamples).
4. Creates the metadata protection for each encrypted Block/SimpleBlock,
with those informations: KeyID (extracted from ContentEncryption element),
IV and partitioning format.
5. Adds a new caps for WebM encrypted content named "application/x-webm-enc",
with the following new fields:
"encryption-algorithm": The encryption algorithm used.
values: "None", "DES", "3DES", "Twofish", "Blowfish", "AES".
"encoding-scope": The field that describes which Elements have been modified.
Values: "frame", "codec-data", "next-content".
"cipher-mode": The cipher mode used in the encryption.
Values: "None", "CTR".
https://bugzilla.gnome.org/show_bug.cgi?id=765275
0432826950