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

src/SDL.c

@@ -400,7 +400,7 @@ int SDL_InitSubSystem(Uint32 flags)
 
     (void)flags_initialized; /* make static analysis happy, since this only gets used in error cases. */
 
-    return 0;
+    return SDL_ClearError();
 
 quit_and_error:
     SDL_QuitSubSystem(flags_initialized);

src/SDL_error.c

@@ -78,13 +78,14 @@ const char *SDL_GetError(void)
     }
 }
 
-void SDL_ClearError(void)
+int SDL_ClearError(void)
 {
     SDL_error *error = SDL_GetErrBuf(SDL_FALSE);
 
     if (error) {
         error->error = SDL_ErrorCodeNone;
     }
+    return 0;
 }
 
 /* Very common errors go here */

src/dynapi/SDL_dynapi_procs.h

@@ -107,7 +107,7 @@ SDL_DYNAPI_PROC(void,SDL_CleanupTLS,(void),(),)
 SDL_DYNAPI_PROC(int,SDL_ClearAudioStream,(SDL_AudioStream *a),(a),return)
 SDL_DYNAPI_PROC(int,SDL_ClearClipboardData,(void),(),return)
 SDL_DYNAPI_PROC(void,SDL_ClearComposition,(void),(),)
-SDL_DYNAPI_PROC(void,SDL_ClearError,(void),(),)
+SDL_DYNAPI_PROC(int,SDL_ClearError,(void),(),return)
 SDL_DYNAPI_PROC(int,SDL_ClearProperty,(SDL_PropertiesID a, const char *b),(a,b),return)
 SDL_DYNAPI_PROC(void,SDL_CloseAudioDevice,(SDL_AudioDeviceID a),(a),)
 SDL_DYNAPI_PROC(void,SDL_CloseCamera,(SDL_Camera *a),(a),)