Take the print config by reference while simplifying extrusions

simplify_path, simplify_multi_path and simplify_loop each copied the whole
PrintConfig, 10520 bytes with around 200 heap owning members, once per extrusion
entity, from inside the parallel_for over all layers. LayerRegion.cpp already
binds a reference a few hundred lines above. Three more sites in Layer.cpp do the
same thing.

The tree support changes in the same area: draw_circles() dropped layers from an
owning vector without deleting them, so every re-slice leaked the layers above
the topmost overhang. std::stable_partition rather than std::remove_if, because
remove_if leaves moved-from duplicates of kept pointers in the tail and deleting
those would be a double free. The collision and avoidance accessors returned
ExPolygons by value out of a cache that hands them out by reference, several
times per node per layer. The preview cache is released when the support step
finishes instead of at the start of the next slice; nothing reads it in between.

set_shared_object() now releases the layers the object still owns before it
starts aliasing another object's. Both clear functions are no-ops once the alias
is set, so this cannot free the sharee's layers.

PrintObjectSlice.cpp: eps was a function local static inside a parallel_for, so
slice_closing_radius was latched once per process. This one changes output for
projects that override it per object, or on a re-slice after editing it. The old
behaviour was undefined: whichever object won the race set it for the session.

src/libslic3r/PrintObject.cpp

@@ -987,6 +987,9 @@ void PrintObject::generate_support_material()
             this->_generate_support_material();
             m_print->throw_if_canceled();
         }
+        // Orca: the tree support collision/avoidance caches and support nodes are only used while this step runs
+        // (detect_overhangs() rebuilds them from scratch), so don't keep them resident until the next slice.
+        this->clear_tree_support_preview_cache();
         this->set_done(posSupportMaterial);
     }
 }