Remove crossing validation asserts in EllipticalArc

The intersection code in EllipticalArc involves a validation step which makes sure that the candidate intersections found are numerically valid. Any invalid intersections are removed by this step.

This MR makes sure that the appearance (and removal) of such invalid intersections does not raise assertions, which could otherwise crash Inkscape. Additionally, the filter routine EllipticalArc::_filterIntersections() is refactored, making it an actual filter (returning the filtered list of intersections).

Reference: inbox#7534 (closed)