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/Fill/FillRectilinear.cpp

@@ -3090,10 +3090,11 @@ bool FillRectilinear::fill_surface_trapezoidal(
     case 0: // Grid / Trapezoidal
     {
         // Generate a non-crossing trapezoidal pattern to avoid overextrusion at intersections when `multiline > 1`.
-        //         P2--P3
-        //        /      \
-        //  P0_P1/        \P4_
-        //
+        /*
+         *         P2--P3
+         *        /      \
+         *  P0_P1/        \P4_
+         */
         // P0xP1x=P4xP0x=d1/2
         // P2xP3x=d1
         // P1yP2y=P2yP3y=d2
@@ -3171,10 +3172,12 @@ bool FillRectilinear::fill_surface_trapezoidal(
     case 1: // Triangular
     {
         // Generate a non-crossing trapezoidal pattern with a base line below.
-        //      P1-P2
-        //     /     \
-        //  P0/       \P3_P4
-        //  ----------------
+        /*
+         *      P1-P2
+         *     /     \
+         *  P0/       \P3_P4
+         *  ----------------
+         */
         // P1xP2x=P3xP4x=d2
         // P0yP1y=P2yP3y=h-2d1
         //

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);