Added thread-safe environment functions

Also marked the existing functions as unsafe, as they can cause crashes if used in multi-threaded applications.

As a bonus, since the new functions are hashtable based, hint environment lookups are much faster.

src/SDL.c

@@ -255,12 +255,14 @@ void SDL_InitMainThread(void)
     SDL_InitLog();
     SDL_InitProperties();
     SDL_GetGlobalProperties();
+    SDL_GetEnvironment();
     SDL_InitHints();
 }
 
 static void SDL_QuitMainThread(void)
 {
     SDL_QuitHints();
+    SDL_CleanupEnvironment();
     SDL_QuitProperties();
     SDL_QuitLog();
     SDL_QuitFilesystem();