Multithreading 8/N: Add support to calling OpenAL from pthreads

[juj]

Similar to #5532, this extends OpenAL API to be coherently visible in all running threads.

We may want to defer landing this until I have had time to address #5367, since these will overlap. Redoing this PR is quite easy, so can layer this on top of #5367 after it is taken care of.

[juj]

This will need #5531 to land first.

[jpernst]

Interesting. Is there a way to detect which thread a request has been proxied from? If so, I could use that to implement ALC_EXT_thread_local_context after this lands for more GL-like context behavior.

[juj]

Interesting. Is there a way to detect which thread a request has been proxied from?

Actually, yes! That kind of mechanism will be introduced in a couple of PRs later, where it is needed for other purposes as well. JS code will be able to detect if they are currently processing a proxied call on behalf of another thread, and get the thread pointer of the thread they are performing the call for.

[kripken]

lgtm

for this and the SDL one, can we add a pthreads test?

[juj]

for this and the SDL one, can we add a pthreads test?

Yeah, I'll mark that down as a followup, since I'm juggling some 20 branches here at the moment.