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.

include/SDL3/SDL_gamepad.h

@@ -268,7 +268,7 @@ extern DECLSPEC int SDLCALL SDL_AddGamepadMapping(const char *mapping);
  * constrained environment.
  *
  * \param src the data stream for the mappings to be added
- * \param freesrc if SDL_TRUE, calls SDL_RWclose() on `src` before returning,
+ * \param freesrc if SDL_TRUE, calls SDL_DestroyRW() on `src` before returning,
  *                even in the case of an error
  * \returns the number of mappings added or -1 on error; call SDL_GetError()
  *          for more information.