testgpu_simple_clear: Fixed AppInit return values.

test/testgpu_simple_clear.c

@@ -47,12 +47,12 @@ SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
 	gpu_device = SDL_CreateGPUDevice(TESTGPU_SUPPORTED_FORMATS, SDL_TRUE, NULL);
 	if (!gpu_device) {
 		SDL_Log("SDL_CreateGPUDevice failed: %s", SDL_GetError());
-		return -1;
+		return SDL_APP_FAILURE;
 	}
 
 	if (!SDL_ClaimWindowForGPUDevice(gpu_device, state->windows[0])) {
 		SDL_Log("SDL_ClaimWindowForGPUDevice failed: %s", SDL_GetError());
-		return -1;
+		return SDL_APP_FAILURE;
 	}
 
     mode = SDL_GetCurrentDisplayMode(SDL_GetPrimaryDisplay());