tableau.svg: group QR code with a (white, "invisible") background

tableau.svg: group QR code with a (white, "invisible") background

The QR code is a path, so the "click" handler that we want to write
for it will only work if the thin black squiggle is tapped. To work
around that, we add a square white background behind the squiggle that
is the same color as the ticket. Afterwards, the "QR code" consists of
the black squiggle and the square; you can click on either.