Figure out support for AV1

[ferjm]

We need to figure out the current status of AV1 support in GStreamer, if there's anything missing there and the steps that we need to do to add its support to servo-media.

CC @ceyusa

[jdm]

1.14.0 incorporates libaom according to https://gstreamer.freedesktop.org/releases/1.14/.

[sdroege]

There's working support via libaom in GStreamer. Someone might want to write a new plugin around dav1d though, for performance reasons.

[philn]

https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/blob/master/video/dav1d/src/dav1ddec.rs :)