LibVideo: Add StartingStateHandler to prepare the first frames of video

This new state handler will retrieve and display the first frame, while
ensuring that playback can start as soon as possible by buffering two
frames on top of the first frame for the PlayingStateHandler to set its
next frame timer by.

Userland/Libraries/LibVideo/PlaybackManager.h

@@ -114,6 +114,7 @@ public:
 
 private:
     class PlaybackStateHandler;
+    class StartingStateHandler;
     class PlayingStateHandler;
     class PausedStateHandler;
     class BufferingStateHandler;