S2Cell(S2CellId(p)).Contains(p) requires expansion of (1.125 + eps) * eps

`S2Cell(S2CellId(p)).Contains(p) ` should always hold, and there is a test to verify this, but there are counterexamples, causing flaky tests.

https://github.com/google/s2geometry/blob/ead4b7a4bfca58067bd473de04f2f9cc5da70250/src/s2/s2cell_test.cc#L494-L500

Is the expansion amount just wrong here?

https://github.com/google/s2geometry/blob/ead4b7a4bfca58067bd473de04f2f9cc5da70250/src/s2/s2cell.cc#L299-L306

We can empirically determine the amount that makes it pass (I think it was around 1.5 ε, maybe smaller), but it would be good to have the correct error analysis.

See also discussion in https://github.com/golang/geo/issues/120.

@ericveach