Substract operation of two paths results in open path error message

[iconexperience]

This simple subtract operation of two paths results in an "Boolean operation resulted in open path" message and an incorrect result.

var p1 =  new Path({segments:[
                [522, 352, 0, 0, 0, 0],
                [500, 400, 0, 0, 0, 0],
                [480, 400, 0, 0, 0, 0],
                [448, 448, 0, 0, 0, 0],
                [100, 448, 0, 0, 0, 0],
                [100, 352, 0, 0, 0, 0]],
                closed:true});
var p2 = new Path.Rectangle(250, 300, 100, 200);
var result = p1.subtract(p2);

Here is the sketch.

On the screenshot below I have highlighted an important area with a circle:

The horizontal line in the highlighted area seems to be part of the cause for this bug. If the line is changed to a non-horizontal line, the error message disappears and a correct result is computed. Here is a sketch to test.

Note that this bug is different from #835, as this occurs on a subtraction between two paths.

[iconexperience]

Everything seems to work well until tracePaths(), so that's where we need to look into.
When entering tracePaths() the _winding attribute of three segments is already different if we change the horizontal line to non-horizontal. So we must investigate what happens previously.

[iconexperience]

I am almost 100% sure that this bug is a duplicate of #819, because the difference in winding gets reported for points that have the same y value as the horizontal line, just in the example in #819. So let's finally fix the getWinding() function.

[iconexperience]

Closing this as duplicate of #819.