Multiply the longitude offset term by `sign_lon_diff` when calculating
equator crossings in `sjoberg_geodesic::lon_of_equator_intersection()`.
Without this multiplication, when `sign_lon_diff` is negative, the offset
is applied in the wrong direction, leading to a wrong intersection
longitude (e.g. returning a coordinate outside of the segments' bounds).
This in turn causes overlay/intersection algorithms to miss crossings on
the boundary.
Including regression test for sjoberg_intersection
Fixes#1482
Frédéric BRIOL