matroska-mux: Add GstVideoCodecAlpha support
WebM alpha mode is now supported for demuxing and decoding, the next logical step would be to support encoding and muxing. For this to happen, we'll need an encoder wrapper similiar to what is used for decoders. Inside the bin, we'll need, alphasplit, to separate the colors and the alpha planes. This implementation will likely need to behave differently depending on the encoder used. Then two encoder instances (unmodified) can be used and a codecalphamux element will be used to put the alpha channel in the color GstBuffer using GstVideoCodecAlphaMeta (and of course update the caps).
Optionally, encoder API can supports alpha internally can simply expose such support without wrapper.
With this in place, Matroska mixer will need to add the alpha mode tag, and attach the auxiliary alpha data.