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/test/SDL_test_common.c

@@ -1913,7 +1913,7 @@ static const void *SDLTest_ScreenShotClipboardProvider(void *context, const char
                     image = NULL;
                 }
             }
-            SDL_RWclose(file);
+            SDL_DestroyRW(file);
 
             if (image) {
                 data->image = image;
@@ -1984,7 +1984,7 @@ static void SDLTest_PasteScreenShot(void)
             if (file) {
                 SDL_Log("Writing clipboard image to %s", filename);
                 SDL_RWwrite(file, data, size);
-                SDL_RWclose(file);
+                SDL_DestroyRW(file);
             }
             SDL_free(data);
             return;