_addAdjoiningFace( eyeVertex, horizonEdge ) {

		// all the half edges are created in ccw order thus the face is always pointing outside the hull

		const face = Face.create( eyeVertex, horizonEdge.tail(), horizonEdge.head() );

		this.faces.push( face );

		// join face.getEdge( - 1 ) with the horizon's opposite edge face.getEdge( - 1 ) = face.getEdge( 2 )

		face.getEdge( - 1 ).setTwin( horizonEdge.twin );

		return face.getEdge( 0 ); // the half edge whose vertex is the eyeVertex


	}
Creates a face with the vertices 'eyeVertex.point', 'horizonEdge.tail' and 'horizonEdge.head'
in CCW order. All the half edges are created in CCW order thus the face is always pointing
outside the hull.

@private
@param {VertexNode} eyeVertex - The vertex that is added to the hull.
@param {HalfEdge} horizonEdge - A single edge of the horizon.
@return {HalfEdge} The half edge whose vertex is the eyeVertex.