Reworked the Archimedean Chords, Hilbert and Octagram Spiral infill
generators to
1) Generate solid infill as not aligned with other solid infill layers.
This may surprise some users as the pattern over multiple islands
will be different, maybe not that nice. This may change in the future.
2) Sparse infill is always aligned and generated over the whole object,
however newly the generated lines are trimmed with a snug bounding box
while being generated.
3) For Archimedean chords the accuracy was not applied correctly, leading
to higher accuracy for dense infill and lower accuracy for sparse infill.
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