OverlayEdge: Don't skip first point when adding coordinates

[dbaston]

The isFirstEdge logic in OverlayEdge::addCoordinates appears to be backwards. It works out OK because it is only used to construct rings, and the dropped point ends up being added later on, with the ring being "rotated" by one vertex.

However, problems arise when extending this function to handle coordinates from CircularString because dropping the first point of the first arc causes a control point to be interpreted as an endpoint.

Fixes #865

[dr-jts]

Nice. This fixes #865.

It would be good to add a unit test in OverlayNGTest:

  /**
   * Tests that overlay produces stable (non-rotated) results.
   * See https://github.com/locationtech/jts/issues/865
   */
  public void testRingsNonRotated() {
    Geometry a = read("POLYGON ((1 9, 9 9, 9 1, 1 1, 1 9))");
    Geometry actual = intersection(a, a);
    checkEqualExact(a, actual);
  }