This warning isn't enabled by default by either -Wall or -Wextra, and
for a good reason. When I do that, it fires also for all uses of
Platform::Application, basically any subclass causes that warning to be
printed as well, even if given subclass is used just once, in an
anonymous namespace, and only through the MAGNUM_APPLICATION_MAIN()
macro. The only solution on Clang is to make such subclasses `final`,
but on GCC not even that helps and one is forced to really make the base
destructor virtual. Which doesn't achieve anything, only adds yet
another entry to the vtable.
So, in the SceneGraph it fixes one particular use case where the warning
was triggered, and since the fix isn't so invasive I'm fine with that.
For Platform I have no idea how an acceptable fix would look like, so I
hope nobody needs any of that anytime soon.
See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102168, especially
the part saying "This warning should not be used.".
Vladimír Vondruš