Remove almost all instances of "volatile" keyword.

As Tiffany pointed out in Bugzilla, volatile is not useful for thread safety:

https://software.intel.com/en-us/blogs/2007/11/30/volatile-almost-useless-for-multi-threaded-programming/

Some of these volatiles didn't need to be, some were otherwise protected by
spinlocks or mutexes, and some got moved over to SDL_atomic_t data, etc.

Fixes Bugzilla #3220.

src/dynapi/SDL_dynapi.c

@@ -293,7 +293,7 @@ SDL_InitDynamicAPI(void)
      *  SDL_CreateThread() would also call this function before building the
      *  new thread).
      */
-    static volatile SDL_bool already_initialized = SDL_FALSE;
+    static SDL_bool already_initialized = SDL_FALSE;
 
     /* SDL_AtomicLock calls SDL mutex functions to emulate if
        SDL_ATOMIC_DISABLED, which we can't do here, so in such a