todo: Add secondary bug not fixed in last commit

mt-work/todo.txt

@@ -34,6 +34,10 @@ other:
 - ffmpeg with -vsync != 0 breaks A/V sync because
 it can't handle more than 1 frame of decoder delay.
 This can be seen with H.264 on mainline.
+- Decoding single-frame files with 3+ threads never
+outputs anything in ffmpeg. It should stop the initial
+delay and just return a frame immediately if one of the
+first N frames is an EOF packet.
 - Error resilience has to run before ff_report_frame_progress()
 is called.
 - 'make test' must pass. This means pthread emulation