threads: Move SDL's own thread creation to a new internal API.

This allows us to set an explicit stack size (overriding the system default
and the global hint an app might have set), and remove all the macro salsa
for dealing with _beginthreadex and such, as internal threads always set those
to NULL anyhow.

I've taken some guesses on reasonable (and tiny!) stack sizes for our
internal threads, but some of these might turn out to be too small in
practice and need an increase. Most of them are simple functions, though.

src/thread/SDL_thread.c

@@ -423,6 +423,16 @@ SDL_CreateThread(int (SDLCALL * fn) (void *),
 #endif
 }
 
+SDL_Thread *
+SDL_CreateThreadInternal(int (SDLCALL * fn) (void *), const char *name,
+                         const size_t stacksize, void *data) {
+#ifdef SDL_PASSED_BEGINTHREAD_ENDTHREAD
+    return SDL_CreateThreadWithStackSize(fn, name, stacksize, data, NULL, NULL);
+#else
+    return SDL_CreateThreadWithStackSize(fn, name, stacksize, data);
+#endif
+}
+
 SDL_threadID
 SDL_GetThreadID(SDL_Thread * thread)
 {