Thanks to "Sander van Dijk" <a.h.vandijk@gmail.com>, we now have much
better SDL sound support.
Here's the promised cleaned up version of the "double buffer" approach
patch for "snd_sdl.c". I've taken some more time to re-read and test
it this time, and it seems to behave well. All memory that is used by
both the main thread and the SDL audio thread is prefixed with "shm_",
and locking is used to ensure that only one thread accesses it at the
same time.
05bf637466