The old `containsPoint` method was sensitive to failure when looking for ray crossings with nearly vertical edges. These added tests demonstrate the simple cases where `containsPoint` succeeds. The tests also now include a case that covers the failure for polygons with nearly vertical edges. The previous `getX` method (within the `containsPoint` method) and the new one are mathematically equivalent. The updated version performs better in cases using coordinates with many significant figures.
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