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/MeshBoolean.cpp

@@ -352,7 +352,7 @@ void segment(CGALMesh& src, std::vector<CGALMesh>& dst, double smoothing_alpha =
         //}
         //else
         {
-            dst.emplace_back(std::move(CGALMesh(out)));
+            dst.emplace_back(CGALMesh(out));
         }
     }
     //if (mesh_merged.is_empty() == false) {
@@ -371,7 +371,7 @@ std::vector<TriangleMesh> segment(const TriangleMesh& src, double smoothing_alph
     std::vector<TriangleMesh> out_meshes;
     for (auto& outf_cgal_mesh: out_cgal_meshes)
     {
-        out_meshes.emplace_back(std::move(cgal_to_triangle_mesh(outf_cgal_mesh.m)));
+        out_meshes.emplace_back(cgal_to_triangle_mesh(outf_cgal_mesh.m));
     }
 
     return out_meshes;