FindQuads has a resolution dependant pyramid

Instead of scaling 3 times the input image, a pyramid is built until the scaled
images are at least 320px wide.
Also, Canny edge detection is applied on each image of the pyramid, rather than
once on the binary image to be scaled down. This makes The edge detection more
robust to scale.

The performance increases most on high resolution images, which had tags to big
to be taken into account. The results are now comparable to LSD, without the
time penalty.

Before:
Processing times (ms) results, on 1 iterations:
  n    Pixels   Average        SD
  8     76800       3.1       0.7
 16    307200      11.5       5.7
  1   1228800      37.7       0.0
  3   1920000      46.2      12.3
  4   7990272     100.2       9.9
234/573 tags were not detected.

Now:
Processing times (ms) results, on 1 iterations:
  n    Pixels   Average        SD
  8     76800       2.2       0.4
 16    307200      10.2       4.0
  1   1228800      30.2       0.0
  3   1920000      46.5      14.2
  4   7990272     249.3      85.8
111/573 tags were not detected.

LSD:
Processing times (ms) results, on 1 iterations:
  n    Pixels   Average        SD
  8     76800      16.9       6.3
 16    307200      56.0      19.9
  1   1228800     236.2       0.0
  3   1920000     806.3     728.5
  4   7990272    1798.5     285.8
141/573 tags were not detected.
Tweaked FindQuads: +20 true pos, +0 false neg

The tweak increases the tolerance in simplifying the contours into polygons
(possible quads).

Before:
Processing times (ms) results, on 1 iterations:
  n    Pixels   Average        SD
  8     76800       2.2       0.4
 16    307200      10.2       4.0
  1   1228800      30.2       0.0
  3   1920000      46.5      14.2
  4   7990272     249.3      85.8
111/573 tags were not detected.

Now:
Processing times (ms) results, on 1 iterations:
  n    Pixels   Average        SD
  8     76800       2.8       0.7
 16    307200      11.4       4.5
  1   1228800      33.6       0.0
  3   1920000      47.0      12.7
  4   7990272     285.7      98.7
91/573 tags were not detected.
Tweaked FindQuads: +2 true pos, +0 false neg

This tweak has a tiny effect on detection, but not so tiny on processing time...

Before:
Processing times (ms) results, on 1 iterations:
  n    Pixels   Average        SD
  8     76800       2.8       0.7
 16    307200      11.4       4.5
  1   1228800      33.6       0.0
  3   1920000      47.0      12.7
  4   7990272     285.7      98.7
91/573 tags were not detected.

Now:
Processing times (ms) results, on 1 iterations:
  n    Pixels   Average        SD
  8     76800       2.9       0.7
 16    307200      11.8       4.6
  1   1228800      34.2       0.0
  3   1920000      52.5      19.6
  4   7990272     298.4      97.4
89/573 tags were not detected.