shapedet: Draw hough's acumulator in gnuplot

shapedet: Draw hough's acumulator in gnuplot

This is a quick hack, but it should be extremely useful in settings
right parameters of haugh transform (scale and maybe some maximal distance
or minimum number of points).

In production code, the debugging should be turned off!

This patch also converts all constants to millimeters, because I found
it confusing to think about which variable is in which units.