Buggy result on data with co-linear lines

[jahting]

As expected, on data with co-linear lines no resulting triangles are returned. But here is an exception where a result is returned, and it's buggy:

[ [ [ 15, 21 ], [ 3, 21 ], [ 20, 1 ], [ 40, 19 ], [ 25, 19 ], [ 37, 19 ], [ 19, 40 ], [ 1, 21 ] ],
[ [ 4, 21 ], [ 6, 23 ], [ 7, 25 ], [ 9, 21 ] ],
[ [ 5, 21 ], [ 11, 21 ], [ 10, 18 ], [ 8, 17 ] ],
[ [ 27, 19 ], [ 29, 20 ], [ 31, 22 ], [ 33, 19 ] ],
[ [ 28, 19 ], [ 34, 19 ], [ 32, 16 ], [ 30, 15 ] ] ]