rwops: Reworked RWops for SDL3.

- SDL_RWops is now an opaque struct.
- SDL_AllocRW is gone. If an app is creating a custom RWops, they pass the
  function pointers to SDL_CreateRW(), which are stored internally.
- SDL_RWclose is gone, there is only SDL_DestroyRW(), which calls the
  implementation's `->close` method before freeing other things.
- There is only one path to create and use RWops now, so we don't have to
  worry about whether `->close` will call SDL_DestroyRW, or if this will
  risk any Properties not being released, etc.
- SDL_RWFrom* still works as expected, for getting a RWops without having
  to supply your own implementation. Objects from these functions are also
  destroyed with SDL_DestroyRW.
- Lots of other cleanup and SDL3ization of the library code.

src/audio/disk/SDL_diskaudio.c

@@ -68,7 +68,7 @@ static int DISKAUDIO_CaptureFromDevice(SDL_AudioDevice *device, void *buffer, in
         buflen -= br;
         buffer = ((Uint8 *)buffer) + br;
         if (buflen > 0) { // EOF (or error, but whatever).
-            SDL_RWclose(h->io);
+            SDL_DestroyRW(h->io);
             h->io = NULL;
         }
     }
@@ -88,7 +88,7 @@ static void DISKAUDIO_CloseDevice(SDL_AudioDevice *device)
 {
     if (device->hidden) {
         if (device->hidden->io) {
-            SDL_RWclose(device->hidden->io);
+            SDL_DestroyRW(device->hidden->io);
         }
         SDL_free(device->hidden->mixbuf);
         SDL_free(device->hidden);