[openal] HRTF queries

[Chris Robinson]

Hey guys.

I'm currently trying to finish up the ALC_SOFT_HRTF extension, and 
there's a few design considerations I need to tackle. Currently the 
extension is just a simple toggle on the device, and HRTF gets enabled 
if requested and a compatible format can be set. However, it would be 
reasonable to have an API that can enumerate multiple HRTF sets, even 
those that use the same format, so that the app can select specific ones.

Ideally, this API would be able to provide names for the HRTF data sets, 
whether it be a filename or some kind of identifier (like device 
enumeration). And in consideration of loopback devices that work with 
user-specified formats, the app needs to be able to query the format it 
uses.

Are there any ideas for how an API would be? The current OpenAL API 
doesn't seem to be designed with the idea of querying string lists or 
setting one as a device property, so I'd like to get thoughts and ideas 
before throwing stuff together.

And a final thing. Right now, OpenAL Soft has the idea that HRTF does 
not strictly have to be stereo. This strikes me as both reasonable 
(could there be HRTFs that work with more than 2 channels?) and a bit 
silly (OpenAL Soft will only do HRTF with stereo). So I'm probably going 
to get rid of the idea of non-stereo HRTFs internally. However, should 
the public HRTF API consider the possibility of non-stereo HRTF?