Fix corner cutting of `pointsToAvoid`

[robbeman]

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When corner cutting is enabled it was still possible to cut corners of points to avoid. Checking pointsToAvoid in isTileWalkable prevents cutting through these corners.

Not 100% sure if this behaviour is appropriate in all situations or if this should become a separate setting?

[prettymuchbryce]

This makes sense to me. Do you mind adding a new unit test which succeeds with this change, but would fail previously?

[robbeman]

I've added the unit test. When commenting out my previous change it will fail by finding a path through the corners of the points marked to avoid.