Fixed a numerical issue at tile exit.

The low point at tile minimum elevation was computed with a tiny
positive error, meaning the point really was above min elevation (at
micrometer level). The line segment between entry and exit stopped just
before traversing the Digital Elevation Model, and an error was
triggered.

The solution was to take some margin when computing the segment
endpoints at entry and exit: we now start above max elevation and end
below min elevation so the intersection should be really on the line.