Make SKIP_FRAMES a compilation flag

The skip_frames flag was a non-configurable runtime flag. Since it is
not exposed to the user, there is no need for a (possible) runtime cost.

For testing purpose, we still want it to be configurable, so make it a
compilation flag.

app/src/decoder.h

@@ -11,7 +11,6 @@ struct decoder {
     TCPsocket video_socket;
     SDL_Thread *thread;
     SDL_mutex *mutex;
-    SDL_bool skip_frames;
 };
 
 SDL_bool decoder_start(struct decoder *decoder);