Bug fix in find_contours

Find_contours: single white pixels could cause an infinite loop due to two uninitialized variables in a code path. This is now fixed with a additional test case.

First release

draw_polygon should be more or less feature complete.

On smaller polygons (< 150 points, 1000x1000 mask) with few intersecting edges, 1 call takes ~ 0.3 ms (opencv 0.1-0.2 ms), a tad slower.
On larger polygons (500 points, 1000x1000 mask) with many intersections , 1 call takes ~ 7 ms (opencv ~ 68ms), much faster.

find_contours is working but the output is very point dense, even on straight lines.

Feedback is welcomed.