Adjust combined infill flow to correct underextrusion

[xiphmont]

Resubmitted without extraneous whitespace changes.

Combined infill currently sets extrusion flow as if implicit extrusion cross-section scales proportionally with layer height. That would be true with a rectangular or cross-sectional extrusion model, but with a rounded rectangle model, the proportion of the extrusion taken by the two semicircles increases and the center rectangle narrows.

This profile change with increased height/thickness causes underextrusion when width and spacing are both kept constant. Delivered density falls below the requested value and, in the worst case, the underextrusion can trigger "extrusion width too small?" errors as the rounded rectangle inverts past a circle.

This change:

1) rescales rounded rectangle width to preserve requested fill density
when fill surface thickness != layer height.

2) relaxes the requirement for aligned spacing in the event requested
density is 100% (treating the fill as solid for Flow calculation, though
preserving it as InternalInfill).