webcam video support

[Antoine Martin]

Currently the wiki/Webcam uses jpeg for each frame, but there is no particular reason why we can't use video encodings or even pass-through:

• capture raw H264 from the webcam if available
• re-inject this directly into the loopback device (requires this patch: ​https://github.com/umlaeute/v4l2loopback/pull/142/commits/8d7a6bc766261276a7cb780fe2f4ab3ac847f1f9 for HEVC and VP9)