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.

test/testutils.c

@@ -44,7 +44,7 @@ GetNearbyFilename(const char *file)
 
         rw = SDL_RWFromFile(path, "rb");
         if (rw) {
-            SDL_RWclose(rw);
+            SDL_DestroyRW(rw);
             return path;
         }