audio: Clean up some CloseDevice() interface details.

- It's now always called if device->hidden isn't NULL, even if OpenDevice()
  failed halfway through. This lets implementation code not have to clean up
  itself on every possible failure point; just return an error and SDL will
  handle it for you.

- Implementations can assume this->hidden != NULL and not check for it.

- implementations don't have to set this->hidden = NULL when done, because
  the caller is always about to free(this).

- Don't reset other fields that are in a block of memory about to be free()'d.

- Implementations all now free things like internal mix buffers last, after
  closing devices and such, to guarantee they definitely aren't in use anymore
  at the point of deallocation.

src/audio/SDL_audio.c

@@ -982,9 +982,8 @@ close_audio_device(SDL_AudioDevice * device)
     if (device->convert.needed) {
         SDL_FreeAudioMem(device->convert.buf);
     }
-    if (device->opened) {
+    if (device->hidden != NULL) {
         current_audio.impl.CloseDevice(device);
-        device->opened = SDL_FALSE;
     }
 
     free_audio_queue(device->buffer_queue_head);
@@ -1193,7 +1192,10 @@ open_audio_device(const char *devname, int iscapture,
         close_audio_device(device);
         return 0;
     }
-    device->opened = SDL_TRUE;
+
+    /* if your target really doesn't need it, set it to 0x1 or something. */
+    /* otherwise, close_audio_device() won't call impl.CloseDevice(). */
+    SDL_assert(device->hidden != NULL);
 
     /* See if we need to do any conversion */
     build_cvt = SDL_FALSE;