video: add support for AV12

video: add support for AV12

AV12 is an internally conceived format that is actually
the combination of NV12 and an alpha plane.

This format is to add to gstreamer's webM
transparency support for vp8 and vp9. To this end, two
I420 streams are independently decoded simultaneously for
the actual content and the alpha plane respectively
and these are then combined into A420.

Since most hardware decoders output NV12, this patch adds
NV12+A to make the same workflow possible.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1152>