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/SDL_wave.c

@@ -1629,7 +1629,7 @@ static int WaveReadFormat(WaveFile *file)
             return -1;
         }
     } else if (format->encoding == PCM_CODE) {
-        SDL_RWclose(fmtsrc);
+        SDL_DestroyRW(fmtsrc);
         return SDL_SetError("Missing wBitsPerSample field in WAVE fmt chunk");
     }
 
@@ -1649,7 +1649,7 @@ static int WaveReadFormat(WaveFile *file)
 
         /* Extensible header must be at least 22 bytes. */
         if (fmtlen < 40 || format->extsize < 22) {
-            SDL_RWclose(fmtsrc);
+            SDL_DestroyRW(fmtsrc);
             return SDL_SetError("Extensible WAVE header too small");
         }
 
@@ -1661,7 +1661,7 @@ static int WaveReadFormat(WaveFile *file)
         format->encoding = WaveGetFormatGUIDEncoding(format);
     }
 
-    SDL_RWclose(fmtsrc);
+    SDL_DestroyRW(fmtsrc);
 
     return 0;
 }
@@ -2117,7 +2117,7 @@ int SDL_LoadWAV_RW(SDL_RWops *src, SDL_bool freesrc, SDL_AudioSpec *spec, Uint8
     SDL_free(file.decoderdata);
 done:
     if (freesrc && src) {
-        SDL_RWclose(src);
+        SDL_DestroyRW(src);
     }
     return result;
 }