The previous version had several problems (see wine-staging bug 195):
* IDirectSoundBufferImpl_Duplicate doesn't initialize EAX properties
* Various race-conditions between initializing EAX and setting buffer properties
* Missing locks while iterating through the list of sound buffers

To fix problems 2 and 3 it might seem useful to introduce additional locking,
but testing reveals that it slows down the mixer thread too much. Some applications
use a huge amount of buffers, and its a waste of time to update them all immediately.
Thats why this patch adds a 'reverb_update' variable, the mixer thread will
update it later when necessary (-> much faster). Moreover, to avoid unnecessary
locking, we initialize the EAX-specific properties always (for both the device and
the buffers).