Fix StringName double-free

The library accidentally took ownership of a StringName passed in (get_virtual callback).
This caused the destructor to run multiple times, creating UB and random crashes.

Re-enables the StringName destructor, which was disabled in 70da3fe.

Author: Bromeon

godot-core/src/builtin/string_name.rs

@@ -53,7 +53,7 @@ impl GodotFfi for StringName {
 impl_builtin_traits! {
     for StringName {
         Clone => string_name_construct_copy;
-        // Drop => string_name_destroy;
+        Drop => string_name_destroy;
         Eq => string_name_operator_equal;
         Ord => string_name_operator_less;
     }

godot-core/src/registry.rs

@@ -314,8 +314,10 @@ pub mod callbacks {
         _class_user_data: *mut std::ffi::c_void,
         name: sys::GDNativeStringNamePtr,
     ) -> sys::GDNativeExtensionClassCallVirtual {
-        let method_name = StringName::from_string_sys(name);
-        let method_name = method_name.to_string();
+        // This string is not ours, so we cannot call the destructor on it.
+        let borrowed_string = StringName::from_string_sys(name);
+        let method_name = borrowed_string.to_string();
+        std::mem::forget(borrowed_string);
 
         T::__virtual_call(method_name.as_str())
     }