tangent points not properly calculated at longitudes != 0

[Joern Ungermann]

North/southward pointing tangent points are shifted significantly away from the zero meridian.

[Reimar Bauer]

Better approximation for tangent point calculation.

Better would be a proper calculation in a sterographic projection or on a sphere.

Fixes issue in stable, however.

Fix issue #451

→ <<cset d8c775f580d4>>