Changed SDL_ClearError() to always return 0.

This will provide a quick and easy way of clearing the error when a function succeeds, if we want to do that in a more widespread way.

For now we guarantee that SDL_Init() will never have an error set when it returns successfully.

Fixes https://github.com/libsdl-org/SDL/issues/8421

include/SDL3/SDL_error.h

@@ -105,12 +105,14 @@ extern DECLSPEC const char *SDLCALL SDL_GetError(void);
 /**
  * Clear any previous error message for this thread.
  *
+ * \returns 0
+ *
  * \since This function is available since SDL 3.0.0.
  *
  * \sa SDL_GetError
  * \sa SDL_SetError
  */
-extern DECLSPEC void SDLCALL SDL_ClearError(void);
+extern DECLSPEC int SDLCALL SDL_ClearError(void);
 
 /**
  *  \name Internal error functions