After discussion with Sébastien Fourey and David Tschumperlé, it was
decided that a better fix for the edge case raised in #2785 was to add a
keypoint anyway, even if the point and none of its neigbours have a
positive smoothed curvature, yet they have a positive raw curvature. In
such case, we use the local maximum raw curvature instead of the local
maximum smoothed curvature.