[openal] Ideas on an explicit context API

[Ryan C. Gordon]

> that for every
> 
> alFoo(...) / alFooEXT(...)
> 
> function, there would be a matching
> 
> alFooExplicit(ALCcontext*, ...) / alFooExplicitEXT(ALCcontext*, ...)

I like this idea a lot. All the existing entry points just become simple 
wrappers:

     void alFoo(ALint bar) {
         alFooExplicit(alcGetCurrentContext(), bar);
     }

...and the existing implementation moves to the Explicit version, and 
then it's just a matter of cleaning out references to the current 
context from them.

I don't love the word "Explicit" for this, but that's a minor gripe. If 
I come up with a different word, I'll suggest it here for discussion.

The nasty part of this is going to be how to deal with extensions. Do we 
say there's a finite list of known extensions right now, and we'll list 
them all out as having Explicit equivalents if the AL supports both 
Explicit contexts and that extension, but the app is on its own if an 
extension isn't included and doesn't mention its interaction with 
Explicit context support?

--ryan.