core_timing: Use better reference tracking for EventType. (#3159)

* core_timing: Use better reference tracking for EventType.

- Moves ownership of the event to the caller, ensuring we don't fire events for destroyed objects.
- Removes need for unique names - we won't be using this for save states anyways.

src/core/hle/service/nvflinger/nvflinger.h

@@ -103,7 +103,7 @@ private:
     u32 swap_interval = 1;
 
     /// Event that handles screen composition.
-    Core::Timing::EventType* composition_event;
+    std::shared_ptr<Core::Timing::EventType> composition_event;
 
     Core::System& system;
 };