sdl: copy/present only after rendering

32blit-sdl/System.cpp

@@ -295,16 +295,19 @@ int System::update_thread() {
 	while (true) {
 		SDL_SemWait(s_loop_update);
 		if(!running) break;
-		loop();
+		bool rendered = loop();
 		if(!running) break;
-		SDL_PushEvent(&event);
-		SDL_SemWait(s_loop_redraw);
+		if(rendered) {
+			// present on main thread if we need to
+			SDL_PushEvent(&event);
+			SDL_SemWait(s_loop_redraw);
+	    }
 	}
 	SDL_SemPost(s_loop_ended);
 	return 0;
 }
 
-void System::loop() {
+bool System::loop() {
   SDL_LockMutex(m_input);
   blit::buttons = shadow_buttons;
   blit::tilt.x = shadow_tilt[0];
@@ -314,6 +317,8 @@ void System::loop() {
   blit::joystick.y = shadow_joystick[1];
   SDL_UnlockMutex(m_input);
 
+  bool rendered = false;
+
   // only render at 50Hz (main loop runs at 100Hz)
   // however, the emscripten loop (usually) runs at the display refresh rate
   auto time_now = ::now();
@@ -328,12 +333,16 @@ void System::loop() {
       _mode = requested_mode;
       cur_format = requested_format;
     }
+
+	rendered = true;
   }
 
   blit::tick(::now());
   blit_input->rumble_controllers(blit::vibration);
 
   blit_multiplayer->update();
+
+  return rendered;
 }
 
 Uint32 System::mode() {

32blit-sdl/System.hpp

@@ -18,7 +18,7 @@ class System {
 		int update_thread();
 		int timer_thread();
 
-		void loop();
+		bool loop();
 
 		Uint32 mode();
     Uint32 format();