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 MR adds NV12+A to make the same workflow possible together with GL support for converting it to RGBA for the purposes of rendering it on a display.