build: clear 2 warnings - sites GCC reports and Clang does not (#15597)

Both are in our own code and neither shows up in a clang-cl or clang census,
so the Windows and CI matrices have never reported them.

FillRectilinear.cpp draws two trapezoid diagrams whose lines end in a
backslash, which continues a // comment onto the next line. GCC calls that a
multi-line comment. The diagrams are now block comments, where the rule does
not apply, and the drawings are unchanged.

CutObjectBase has a user-provided operator= and a virtual destructor, either
of which deprecates its implicitly generated copy constructor. bbs_3mf.cpp
copies the type through CutObjectInfo. The copy constructor is now declared
and defaulted, leaving the class with no implicit copy member. Move
operations were already suppressed by the user-provided operator=, so nothing
changes there.

src/libslic3r/ObjectID.hpp

@@ -170,6 +170,10 @@ public:
         this->m_check_sum      = rhs.check_sum();
         this->m_connectors_cnt = rhs.connectors_cnt();
     }
+    // A user-declared copy assignment or destructor deprecates the implicitly generated
+    // copy constructor, and this class has both, so declare it rather than rely on it.
+    CutObjectBase(const CutObjectBase &) = default;
+
     CutObjectBase &operator=(const CutObjectBase &other)
     {
         this->copy(other);