[openal] A debug API

[Chris Robinson]

Alongside the Explicit or Direct API, the idea of a debug API has been 
floated around and asked about. The last few days I've been 
experimenting with adding one to OpenAL Soft, essentially replicating 
the GL_KHR_debug API <https://www.khronos.org/opengl/wiki/Debug_Output>. 
About the only things that are missing currently are object labels/names 
and the debug context flag.

The primary question is, is this an acceptable or desired API? I'm more 
than willing to make changes if someone believes it can be made to 
better fit OpenAL itself, for developers debugging with and using 
OpenAL, or even scrap it entirely if there's a preference for something 
different. The current macro and function definitions are here if 
anyone's interested in looking and trying it out:
<https://github.com/kcat/openal-soft/blob/d5e5b211/alc/inprogext.h#L57>
(what's there is subject to change obviously, so shouldn't be used in 
production code, but can be used to try it and give feedback).

If others agree this is the way forward for this, I can also replace the 
SOFT moniker and suffix to macros/types/functions for EXT (or ARB?) to 
make the naming more neutral.

As far as OpenAL Soft's current implementation goes, with debug output 
enabled it will produce a message (stored in the log or invoke the 
callback) for every AL error generated. It'll produce a message for 
alDopplerVelocity and AL_DOPPLER_VELOCITY being deprecated, and 
alcSuspendContext/alcProcessContext having non-portable behavior, too. 
I'll be adding more when I notice where to put them and am able. When 
the debug context bit is implemented, I intend on disabling most debug 
behavior when not set and only produce messages for AL errors (no group 
push/pop functionality, no UB/deprecation/performance/portability/marker 
messages, etc, so the code can be left in and not incur much penalty 
when not on a debugging context, but still able to get standard AL 
errors through a callback or log queue instead of having to poll 
alGetError).

Questions, comments, and suggestions are welcome.
- Chris