Fix single-threaded decode hang when ready_tasks is zero

[intrepidsilence]

Summary

In single-threaded mode (n_threads == 0), after initializing a task, ready_tasks can be zero because the dependency calculation may not account for the current frame being in a transitional state. The code then calls worker_loop directly, but no task is selected because ready_tasks is empty, causing the decoder to hang.

In multithreaded mode, worker threads recalculate ready_tasks after completing each task (line ~572), providing a second chance. In single-threaded mode there is no such recalculation.

Fix

Add a safety net that force-sets the task bit when ready_tasks is unexpectedly zero in single-threaded mode:

if (dec->n_threads) {
    pthread_cond_signal(&dec->task_ready);
} else if (!dec->ready_tasks) {
    // Safety net: force-set task bit when dependency calc yields 0
    dec->ready_tasks |= 1 << task_id;
    dec->worker_loop(dec);
} else {
    dec->worker_loop(dec);
}

Symptom

Decoder hangs indefinitely with 0% CPU on certain MVC streams. The task is initialized but never executed.

Verification

• Tested on commercial 3D Blu-ray MVC streams (130,000+ frame pairs)
• Before fix: decoder hangs after variable number of frames
• After fix: decoder completes all frames successfully
• 85/85 JVT conformance streams continue to pass