Reuse active stream

[mkotsbak]

When capturing from camera, a stream is setup to the video element. If stop() is not called, the stream is left running. To avoid asking user for permission again after a single (once=true) capture, if the stream is active, it could be used directly by _captureToCanvas.

[mkotsbak]

I implemented this in the Scala.Js wrapper: https://github.com/mkotsbak/scalajs-qcode-decoder/blob/master/src/main/scala/io/github/mkotsbak/qcodeDecoder/QCodeDecoder.scala#L43