Adding/Uncommenting elements for Alpha Support
Adding/Uncommenting elements for Alpha Support

Uncommenting Track elements related to BlockAdditional and adding
the new AlphaMode element as specified in the matroska spec here:
http://matroska.org/technical/specs/index.html#AlphaMode

Change-Id: I87895931e8885e4832efa74776ab1bea91a634e2

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