Supporting dense layer anchor at sparse infill lines

[supermerill]

In v2.2.53, this still seems suboptimal... Is there any reason we can't have a mode like automatic, but instead of expanding the area to a perimeter line, it simply expands to any line on the layer below (including an infill line).

That way, the area will never be expanded beyond the infill line spacing.

For some infill patterns, there might be certain angles where it is a long distance to the next infill line in the worst case. In that case, a different direction for the dense layer support should be chosen.

The algorithm seems simple on the surface, but I guess implementation is harder than it looks...

Originally posted by @Hello1024 in #507 (comment)

[supermerill]

just a note: infill are printed in parallel, so you don't have the "previous layer infill lines".