Fix data race UB (audio thread pushing to its own message queue) triggered by WAV export

[nyanpasu64]

In a previous PR #137, I replaced audio thread communication with inter-thread message queues to prevent blocking the audio thread and interrupting playback. Unfortunately I missed a case where when you perform WAV export, the audio thread itself is pushing into the message queue intended for GUI-to-audio commands. This results in a UB data race on the lock-free ring buffer queue, which could result in missing or invalid commands being processed.

Separately I've noticed crashes upon WAV export where m_bRendering is true, but m_pWaveFile is null while calling m_pWaveFile->WriteWave(). I don't know how it happens, but it may be related to the above UB.

I'm fixing the data race by introducing a separate object for messages sent from the audio thread to itself. Hopefully it fixes the WAV export crash, but it's intermittent and I can't reproduce it on demand on the old code, nor prove that this change fixes it.

Changes in this PR:

• Fix data race UB (audio thread pushing to its own message queue) triggered by WAV export

[Gumball2415]

looks good to me