Fix of safety_offset() after ClipperUtils refactoring.

Fixes Solid infill where there should be none #6482
Also the safety offsetting was revised to be enabled only where needed,
the "do safety offset" is now easy to discover by
a new ApplySafetyOffset::Yes enum, and safety offset over union, which
is better done by offset() / offset_ex() has been replaced with
new union_safety_offset() / union_safety_offset_ex() functions, which
better convey their meaning and which could be better optimized than
union() with the safety offset applied.

src/libslic3r/SLA/SupportPointGenerator.hpp

@@ -90,7 +90,7 @@ public:
         float overlap_area(const Structure &rhs) const { 
             double out = 0.;
             if (this->bbox.overlap(rhs.bbox)) {
-                Polygons polys = intersection(*this->polygon, *rhs.polygon, false);
+                Polygons polys = intersection(*this->polygon, *rhs.polygon);
                 for (const Polygon &poly : polys)
                     out += poly.area();
             }