Improve logging performance and make log priorities thread-safe

Fixes https://github.com/libsdl-org/SDL/issues/9679

test/testrelative.c

@@ -138,9 +138,6 @@ static void loop(void)
 
 int main(int argc, char *argv[])
 {
-    /* Enable standard application logging */
-    SDL_SetLogPriority(SDL_LOG_CATEGORY_APPLICATION, SDL_LOG_PRIORITY_INFO);
-
     /* Initialize test framework */
     state = SDLTest_CommonCreateState(argv, SDL_INIT_VIDEO);
     if (!state) {