Contours: Custom grab snaps incorrectly

[jlampel]

In RF3, it was possible to click and drag on a selected loop to slide it up and down the mesh. In RF4, it looks like that was added but is currently broken. Moving using the same snapping settings actually works fine in Blender's regular move tool, so it suggests that the issue is with the custom grab in RF, and specifically for the nearest surface snapping that Contours uses.