some cleanups and use videoframe API

[ceyusa]

• Some cleanups to avoid multiple versions of some dependencies
• Use VideoFrame from gstreamer-video (adding a new dependency). Also, If I understand correctly, this will save a copy of the frame.

[ceyusa]

@ferjm r?

[sdroege]

This still copies like it did before by requiring to have a Vec<u8> in the API. Ideally you would change the API so that it takes an Arc<AsRef<[u8]>> instead: in that case you could simply pass the return value of buffer.into_mapped_buffer_readable() in there.

My old servo branch had it implemented without copying similar to that.