build: clear 50 warnings - pessimizing moves and null checks that cannot fail (#15408)

* build: remove std::move that blocks copy elision

std::move wrapped around a temporary, or around a local being returned,
stops the compiler constructing it in place. Each edit is the fix clang
suggests, which is to delete the std::move call and keep its argument.

Three of the 39 sites save a move, the two return std::move(local) in
Print.cpp and TreeSupport.cpp:2749. The rest are equivalent either way
and match how the codebase already writes this elsewhere.

Clears 39 -Wpessimizing-move warnings.

* build: drop null checks on references and this

A reference cannot be bound to null and this cannot be null, so the
compiler folds these conditions to true and drops the guard. Seven are
if (&bitmap && bitmap.IsOk()), where IsOk() already does the work; two
test this directly. The guarded code runs either way, so removing the
dead operand changes nothing.

Clears 11 -Wundefined-bool-conversion warnings.

src/libslic3r/Format/STEP.cpp

@@ -596,7 +596,7 @@ Step::Step_Status Step::mesh(Model* model,
                     for (Standard_Integer aNodeIter = 1; aNodeIter <= aTriangulation->NbNodes(); ++aNodeIter) {
                         gp_Pnt aPnt = aTriangulation->Node(aNodeIter);
                         aPnt.Transform(aTrsf);
-                        points.emplace_back(std::move(Vec3f(aPnt.X(), aPnt.Y(), aPnt.Z())));
+                        points.emplace_back(Vec3f(aPnt.X(), aPnt.Y(), aPnt.Z()));
                     }
                     // BBS: copy triangles
                     const TopAbs_Orientation anOrientation = anExpSF.Current().Orientation();

src/libslic3r/Format/bbs_3mf.cpp

@@ -8963,7 +8963,7 @@ private:
         BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << " inital and interval = " << m_interval;
         m_next_backup = boost::get_system_time() + boost::posix_time::seconds(m_interval);
         boost::unique_lock lock(m_mutex);
-        m_thread = std::move(boost::thread(boost::ref(*this)));
+        m_thread = boost::thread(boost::ref(*this));
     }
 
     ~_BBS_Backup_Manager() {

src/libslic3r/Format/svg.cpp

@@ -352,7 +352,7 @@ bool load_svg(const char *path, Model *model, std::string &message)
             for (Standard_Integer aNodeIter = 1; aNodeIter <= aTriangulation->NbNodes(); ++aNodeIter) {
                 gp_Pnt aPnt = aTriangulation->Node(aNodeIter);
                 aPnt.Transform(aTrsf);
-                points.emplace_back(std::move(Vec3f(aPnt.X(), aPnt.Y(), aPnt.Z())));
+                points.emplace_back(Vec3f(aPnt.X(), aPnt.Y(), aPnt.Z()));
             }
             // BBS: copy triangles
             const TopAbs_Orientation anOrientation = anExpSF.Current().Orientation();