Fix DirectSound's primary buffer Volume and Pan handling

[Francois Gouget]

Currently the way DirectSound handles Volume and Pan is wrong:
  * whenever a primary buffer is created (e.g. in DirectSoundCreate()) 
we initialize an internal variable that sets the volume to the max and 
pan to the center.
  * we also call IDsDriverBuffer_SetVolumePan() or waveOutSetVolume() to 
set the system volume to that value. On my machine this has the same 
effect as
    amixer set PCM 100%
  * we ignore changes in the volume or balance made outside of 
DirectSound. So if the user does:
    amixer set PCM 50%
    And then the application calls GetVolume() it will still get 0 (i.e. 
max volume)

This is not what happens on Windows. On Windows primary buffers don't 
have their internal volume. Instead SetVolume()/SetPan() and 
GetVolume()/GetPan() query directly the underlying volume.

I hacked the dsound.c test to verify this. Basically, now it does the 
following:
  * it calls GetVolume() and GetPan() and prints the results
  * it sleeps for 20 seconds
  * during these 20 seconds, start Windows' mixer control, and change 
the 'Wave Sound' (translated from French) volume and/or balance.
  * the test then calls GetVolume() and GetPan() and prints the results 
again

Running the test we can see that the second time DierctSound returns the 
new volume:

dsound.c:771:Primary volume=-566 pan=0
dsound.c:776:Primary volume=-566 pan=0
--- the test sleeps here while we change the volume ---
dsound.c:782:Primary volume=-1438 pan=0

I attached the dsound.c test patch so this can be cross-checked. You 
will note that I also played with the secondary buffers to check the 
effect changing the volume on a secondary buffer would have on the 
system volume: it has none but Wine already does the right thing (a bit 
surprisingly).
(for secondary buffers SetVolume() has a call to 
IDsDriverBuffer_SetVolumePan() but that branch is not taken so we do a 
DSOUND_ForceRemix() instead. I'm not sure if we should ever go through 
this IDsDriverBuffer_SetVolumePan() branch)

I also patched DirectSound to fix the problem. The one thing that 
bothers me is that I did not find the equivalent of an 
IDsDriverBuffer_GetVolumePan(). I.e. we can use the driver to set the 
volume but not to query it. That's pretty strange. I wonder how 
DirectSound gets the system volume on Windows.

In Wine what I did is simply use waveOutGetVolume() on dsound->hwo. Then 
I pass this through DSOUND_AmpFactorToVolPan() to convert these 
'AmpFactors' into the regular Vol/Pan used by DirectSound.

Comment and improvement suggestions welcome.
If nobody objects I'll submit this to wine-patches tomorrow.