Fix inconsistency between viewfinder qrbox and actual feed in canvas

[mebjas]

• Fixed the video size issue in issues/21
• Removed fixed height of viewfinder, now the height is based on the video stream. The width is honored if the input element has a default width. Otherwise default width is applied.
• If config.qrbox is greater than derived height, the config is ignored & no shading is applied.
• The sequence of steps have changed
  • First we get the video feed from the selected camera
  • Then we render video
  • Then based on the height of the video we set the canvas and start scanning.
• For file scanning, if the container element has some height or width both are honored.
  Otherwise default width is applied and height is derived from the image.

[mebjas]

This should fix the inconsistency