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/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();