sdl: fix exiting while waiting for a render to be presented

This would sometimes trigger the "user code frozen" message at exit, in some cases crashing when the semaphores get destroyed whilst the update thread is waiting on them.

32blit-sdl/System.cpp

diff --git a/32blit-sdl/System.cpp b/32blit-sdl/System.cpp
index dd13bd8..cc698c2 100644
--- a/32blit-sdl/System.cpp
+++ b/32blit-sdl/System.cpp
@@ -445,16 +445,19 @@
 }
 
 void System::stop() {
-	int returnValue;
-	running = false;
+  int returnValue;
+  running = false;
 
-	if(SDL_SemWaitTimeout(s_loop_ended, 500)) {
-		std::cerr << "User code appears to have frozen. Detaching thread." << std::endl;
-		SDL_DetachThread(t_system_loop);
-	} else {
-		SDL_WaitThread(t_system_loop, &returnValue);
-	}
+  // make sure the update thread is not waiting for a render to complete
+  SDL_SemPost(s_loop_redraw);
 
-	SDL_SemPost(s_timer_stop);
-	SDL_WaitThread(t_system_timer, &returnValue);
+  if(SDL_SemWaitTimeout(s_loop_ended, 500)) {
+    std::cerr << "User code appears to have frozen. Detaching thread." << std::endl;
+    SDL_DetachThread(t_system_loop);
+  } else {
+    SDL_WaitThread(t_system_loop, &returnValue);
+  }
+
+  SDL_SemPost(s_timer_stop);
+  SDL_WaitThread(t_system_timer, &returnValue);
 }