calculateSourceExtent() {
    const extent = createEmpty();

    this.triangles_.forEach(function (triangle, i, arr) {
      const src = triangle.source;
      extendCoordinate(extent, src[0]);
      extendCoordinate(extent, src[1]);
      extendCoordinate(extent, src[2]);
    });

    return extent;
  }
Calculates extent of the `source` coordinates from all the triangles.

@return {import("../extent.js").Extent} Calculated extent.