Visualization for the cansee command

[dodie]

The "cansee" command tells you if a point in the 3D space is visible from the viewpoint. Currently it prints the yes/no result to the standard output.

Add visualization to this feature.

Ideas:

• Show the two points and a red/green line between them in a relief map.
  

• If the two points are visible, show a contour image, rendered from the first point, looking to the second point, add a marker to the contour image for the second coordinate pair.
  

• Modify the command to only take one coordinate pair, and render a relief map that shows all visible points from the given coordinates.