Note that the loop and cell *do not* contain exactly the same set of
points, because S2Loop and S2Cell have slightly different definitions of
point containment. For example, an S2Cell vertex is contained by all
four neighboring S2Cells, but it is contained by exactly one of four
S2Loops constructed from those cells. As another example, the S2Cell
coverings of "cell" and "S2Loop(cell)" will be different, because the
loop contains points on its boundary that actually belong to other cells
(i.e., the covering will include a layer of neighboring cells).
Construct a loop corresponding to the given cell.
Note that the loop and cell *do not* contain exactly the same set of points, because S2Loop and S2Cell have slightly different definitions of point containment. For example, an S2Cell vertex is contained by all four neighboring S2Cells, but it is contained by exactly one of four S2Loops constructed from those cells. As another example, the S2Cell coverings of "cell" and "S2Loop(cell)" will be different, because the loop contains points on its boundary that actually belong to other cells (i.e., the covering will include a layer of neighboring cells).