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/testresample.c

@@ -141,7 +141,7 @@ int main(int argc, char **argv)
     SDL_WriteU32LE(io, dst_len);                                /* size */
     SDL_RWwrite(io, dst_buf, dst_len);
 
-    if (SDL_RWclose(io) == -1) {
+    if (SDL_DestroyRW(io) == -1) {
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "fclose('%s') failed: %s\n", file_out, SDL_GetError());
         ret = 6;
         goto end;