Scaling the video as it is drawn to the canvas.

When you draw the video to the canvas you have to tell it the size of
the canvas so that it can scale the video appropriately and is
analysing the whole frame of the video when looking for QR codes.

My use-case is a responsive website where the canvas is the size of the 
available screen width. On narrow screens without this code you would
only see a crop of the video...