IDeckLinkVideoOutputCallback haven't virt dtor

IDeckLinkVideoOutputCallback doesn't have a virtual desctructor in
Windows, thus its subclass destructor cannot be marked override.

src/video_display/decklink.cpp

@@ -152,7 +152,7 @@ class PlaybackDelegate : public IDeckLinkVideoOutputCallback // , public IDeckLi
             m_deckLinkOutput = ido;
         }
         void Reset();
-        ~PlaybackDelegate() override {
+        virtual ~PlaybackDelegate() {
                 Reset();
         };
         void PrintStats();