perf: skip estimating curled walls when nothing reads them
The curled extrusion estimate ran whenever a region had overhang speed on,
which is the default, but only the slowdown for curled perimeters reads the
curled lines it produces, and that slowdown is off by default. The step now
also requires a region with the slowdown on, and clears the curled lines
when it skips the estimate, so none are left from an earlier slice.
Also fixes stale fan commands due to the stale curled lines on the reused layers.
Kris Austin
90ac58d3cd