Use buffered area polygon for extended visibility graphs

[pablohoch]

Fixes an issue with area routing if start and/or destination points are close to an area.

When a segment (in the example: the segment between the two points where the red linestring crosses the platform area) is on the outer polygon of the area, the visibility check sometimes didn't work, resulting in crazy routes through the area.

To fix this, we now use a buffer of the outer polygon for visibility checks when extending the visibility graph.

Before:

After:

(Only the gray platform is an area in this example.)

The same problem occured with inner polygons:

Before:

After: