support for different resolutions from fake devices

[fippo]

currently defaults to 640x480 in #4. @jan-ivar suggested

clamp = (min, v, max) => Math.max(min, Math.min(v, max))
canvas.width = (typeof c.video.width === 'object') ? 
      clamp(c.video.width.min || c.video.width.exact,
    (("ideal" in c.video.width)? c.video.width.ideal : 640), c.video.width.max ||
            c.video.width.exact) : c.video.width;