fix (literal) edge cases in fast-polygon operations class

[tyrasd]

Literal edge case here 😆 – Sometimes during the preprocessing step (responsible for optimizing the polygon operations) the intermediate results may contain superfluous points or lines along the clipping edge(s). These additional geometries cause problems further down the processing pipeline. Since these edge artifacts don't contribute anything to the final oshdb query result, they can just be filtered out.

Here is an example of a geometry which can cause such a situation: https://gist.github.com/tyrasd/19ec9399d42dcf76ed7ab16432052afe (found by @redfrexx ~ thanks!).

Checklist

• My code follows the code-style rules and I have checked on the static analyses and benchmark (if applicable) results
• I have commented my code
• I have added sufficient unit tests
• I have updated the CHANGELOG.md

[rtroilo]

the gcPolys list could hold also only a single polygon?
If it makes a difference to handle a Mulitpolygon instead of a Polygon in the following code than it would make sense to do here an additional check.

[tyrasd]

the gcPolys list could hold also only a single polygon?

it can. good point. I'll add a path for the single polygon case.