Rather than have the Audio Unit render callback traverse the queue of wave
headers looking for complete ones, and sending a message to the message thread
for each one it finds, just send one message to tell the message thread to do
that work itself.  The render callback is called in a real-time priority
thread and is expected to return as quickly as possible.

wodHelper_PlayPtrNext is only called when state == WINE_WS_PLAYING.  Also, it
clears dwPartialOffset itself.

wodWrite only calls wodHelper_BeginWaveHdr[Write] with a non-NULL lpWaveHdr
parameter.  wodWrite is not called from the Audio Unit render callback, so it
can use Wine debug channels.

This refactoring prepares the ground for subsequent refactoring of
wodHelper_BeginWaveHdr.

into a DECIMAL instead of a double as an intermediate step.

This also reverts my previous patch as this is the real cause for the
problems with Grand Prix Legends.