Add h264 encoder support

[Icy-Locust]

Just curious why we don't have H.264 encoder support.
This PR contains ffmpeg libav implementation just for Linux.
It is a bare encoder with low efficiency, but better than nothing.
Any comments are welcome!

[CendioOssman]

Thanks for your contribution!

The reason we don't have it is mainly because it is unclear if it's efficient. It needs to provide an advantage over the current encoders for us to add it, much less give it priority.

What kind of performance have you seen in your tests?

[Icy-Locust]

As this article and this issue suggest, it's currently reasonable not to implement H.264 encoding.
Frames sent by this encoder are all I-frames, which means that we don't take full advantage of H.264 codec.
We need to redesign the way encoder processes the buffer, and add bit rate control algorithm.
Close this PR, and any ideas are welcome!