Fix Rust-SDL2#1323: Reset IS_MAIN_THREAD_DECLARED on drop

Author: austindoupnik

src/sdl2/sdl.rs

@@ -209,6 +209,7 @@ impl Drop for SdlDrop {
             unsafe {
                 sys::SDL_Quit();
             }
+            IS_MAIN_THREAD_DECLARED.swap(false, Ordering::SeqCst);
         }
     }
 }