[openal] HRTF sound coloring

[Ian Reed]

Hi Chris,

We are using OpenAL Soft 1.17.2.
Our sound designer noticed some coloring of sounds when playing them 
with HRTF enabled and source relative set to true.
Is having HRTF enabled supposed to affect sounds played with source 
relative set to true and no panning applied?
We don't mind some coloring when the sounds are positioned in 3D, 
obviously HRTF is supposed to change how they sound.
But we want to play some sounds exactly as they were recorded, without 
any coloring.
Is setting source relative to true the right way to play unpanned mono 
and stereo sounds?

I made a test program to demonstrate the issue.
It plays a sound with HRTF, then without, then with, then without, so 
you can hear them side by side.
That is, first and third plays are with HRTF, second and fourth are 
without HRTF.
I'm using your updated API for resetting the device while the program 
runs in order to change the HRTF setting.
I played a mono sound in the first example, then a stereo sound in the 
second.
Both have coloring, but the stereo sound also sounds "wider" when HRTF 
is off.
I recorded them in Audacity and exported as flac:
https://BlindGamers.com/downloads/HRTF mono coloring.flac
https://BlindGamers.com/downloads/HRTF stereo coloring.flac

Here is a snippet of my C# code that uses the OpenTK wrapper around 
OpenAL to create the source:
var s = new Source();
s.SourceID = AL.GenSource();
ALAnnounceError("AL.GenSource");

s.BufferID = CreateAndFillBuffer(path, false);
AL.Source(s.SourceID, ALSourcei.Buffer, s.BufferID);
ALAnnounceError("setting buffer on source");

AL.Source(s.SourceID, ALSourceb.SourceRelative, true);
ALAnnounceError("setting source relative to true");

No errors are reported, and I am happy to provide the full source of the 
test program if you would like.

Thanks,
Ian Reed