codec: Introduce GstVideoCodecAlphaMeta
This meta hold one buffer of the same codec data as the parent memory. This extra frame luma will be used as the alpha values for the final combined frame. This is notably used to support VP8/VP9 alpha as defined in WebM and matroska specification.
This the first MR to enable WebM AlphaMode support, more MRs will be added soon. Please consider this as an early submission as, while it is working for me all this have had very little testing.