gpu: remove Queue.WorkDownCallback lifetime footgun

The previous implementation required the specific Queue struct that
submit() was called on to be valid when the callback is triggered. By
storing a pointer instead, the Queue does not need to be valid, only the
callback itself.

gpu/src/NativeInstance.zig

@@ -902,7 +902,7 @@ const queue_vtable = Queue.VTable{
                     wgpu_queue,
                     signal_value,
                     cCallback,
-                    &queue.on_submitted_work_done,
+                    queue.on_submitted_work_done,
                 );
             }