Gives to video the same appearance as being mirrored.

This is to support audio only feature.
If user disable camera, we show the caller username in the video screen.
If down stream has no video, we also update video area with caller username.

Add a control button in a shared video which allow to stop only this shared video.

Remove code to disable my video camera and keep only audio stream.

Probably useless, but I'm supersticious.

We were inconsistent with stream creation.

It's not useful.

Users sometimes click on the button multiple times.

If a connetion is slow to get established, this would cause it to
be pushed twice, the first time before it is complete.  Thanks to
Roman Koval who noticed the issue.

Apparently, it is possible for an ICE candidate to be generated
before onnegotiationneeded triggers.  Avoid losing a candidate
in that case.