Fix line trim when line begins on polygon edge

[andrewheumann]

BACKGROUND:

• Fixes Line.Trim(Polygon) does not trim correctly with concave polygons. #355

DESCRIPTION:

• Add a better check for whether the line begins outside the polygon
• double-check containment in ambiguous cases

TESTING:

• added TrimLineThatStartsAtPolygonEdge() test, this failed with the old method, succeeds now
• tested on a large range of other conditions similar to the reported case

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This change is 

[ikeough]

Reviewable status: 0 of 5 files reviewed, 1 unresolved discussion (waiting on @andrewheumann and @ikeough)

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src/Elements/Geometry/Line.cs, line 472 at r1 (raw file):

            // Add the segment's starting point.
            intersections.Add(this.Start);
            polygon.Contains(this.Start, out var containment);

Are we going to have a similar problem if the line ends on the polygon as well?

[andrewheumann]

Reviewable status: 0 of 5 files reviewed, 1 unresolved discussion (waiting on @ikeough)

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src/Elements/Geometry/Line.cs, line 472 at r1 (raw file):

Previously, ikeough (Ian Keough) wrote…

Are we going to have a similar problem if the line ends on the polygon as well?

No — it splits the line at all the intersections and alternates its way down from the start to the end. It only matters whether it starts inside or outside.

[ikeough]

Reviewable status: 0 of 5 files reviewed, all discussions resolved