refactor(sonar): more small fixes

- std::format instead of iostream manipulators in to_string/to_hex_str (cpp:S6495);
  output verified identical, negative hex values keep the unsigned form
- remove an empty compound statement (cpp:S108)
- drop an always-false unsigned < 0 branch (cpp:S1768)
- explicit lambda capture (cpp:S3608)
- std::make_unique (cpp:S5950)

rendering/nullrenderer.cpp

@@ -3,7 +3,7 @@
 
 std::unique_ptr<gfx_renderer> null_renderer::create_func()
 {
-    return std::unique_ptr<null_renderer>(new null_renderer());
+    return std::make_unique<null_renderer>();
 }
 
 bool null_renderer::renderer_register = gfx_renderer_factory::get_instance()->register_backend("null", null_renderer::create_func);