thread: Locking mutexes and rwlocks are now void functions.

Almost nothing checks these return values, and there's no reason a valid
lock should fail to operate. The cases where a lock isn't valid (it's a
bogus pointer, it was previously destroyed, a thread is unlocking a lock it
doesn't own, etc) are undefined behavior and always were, and should be
treated as an application bug.

Reference Issue #8096.

test/testlock.c

@@ -80,17 +80,12 @@ Run(void *data)
     SDL_Log("Thread %lu: starting up", SDL_ThreadID());
     while (!SDL_AtomicGet(&doterminate)) {
         SDL_Log("Thread %lu: ready to work\n", SDL_ThreadID());
-        if (SDL_LockMutex(mutex) < 0) {
-            SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Couldn't lock mutex: %s", SDL_GetError());
-            exit(1);
-        }
+        SDL_LockMutex(mutex);
         SDL_Log("Thread %lu: start work!\n", SDL_ThreadID());
         SDL_Delay(1 * worktime);
         SDL_Log("Thread %lu: work done!\n", SDL_ThreadID());
-        if (SDL_UnlockMutex(mutex) < 0) {
-            SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Couldn't unlock mutex: %s", SDL_GetError());
-            exit(1);
-        }
+        SDL_UnlockMutex(mutex);
+
         /* If this sleep isn't done, then threads may starve */
         SDL_Delay(10);
     }