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/Print.cpp

@@ -3790,7 +3790,7 @@ std::vector<Polygons> Print::get_extruder_printable_polygons() const
         Polygons ploys = {Polygon::new_scale(e_printable_area)};
         extruder_printable_polys.emplace_back(ploys);
     }
-    return std::move(extruder_printable_polys);
+    return extruder_printable_polys;
 }
 
 std::vector<Polygons> Print::get_extruder_unprintable_polygons() const
@@ -3803,7 +3803,7 @@ std::vector<Polygons> Print::get_extruder_unprintable_polygons() const
         Polygons ploys = diff(printable_poly, Polygon::new_scale(e_printable_area));
         extruder_unprintable_polys.emplace_back(ploys);
     }
-    return std::move(extruder_unprintable_polys);
+    return extruder_unprintable_polys;
 }
 
 size_t Print::get_extruder_id(unsigned int filament_id) const