Fix issue #242: S-bend disconnect in path_to_waveguide.

[FaresBadrCA]

Issue Description:

In path_to_waveguide, a normal 90-bend followed by an S-bend will create a disconnect in the waveguide. This applies only to S-bends around the third point in the path.

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Cause:

There is an off-by-one error in the check for clearance of S-bends. The third point in the path (P2) is treated as an 'end point', so the check for S-bend viability assumes the segment P1->P2 is all available for use, which is incorrect, since part of that segment is already 'used up' by the previous bend (around P1).

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Solution:
Fix indices in S-bend clearance check so that the third point in a path is an inner point, not an end point.