SDL2 resource destructors segfaults fix

[brainstormi]

Fix for random segfaults in SDL destructors. Discussed in issue #2. SDL_DestroyXXXX and SDL_FreeSurface should be called from the main thread due to the lack of multithreading support in SDL2. Probably not the cleanest or more elegant implementation to merge, but in the worst case, useful as proof of concept solving these segfaults.

[essen]

Thanks. Sounds good. Will merge when I can. :-)