Treat touches outside the viewport as right-clicks

The old method to simulate right click (two-finger touch) still works.

src/sdlmain.cpp

@@ -33,6 +33,7 @@ int main(int argc, char *argv[])
 	}
 
 	SDL_Init(SDL_INIT_VIDEO);
+	SDL_SetHint(SDL_HINT_TOUCH_MOUSE_EVENTS, "0");
 #ifdef __EMSCRIPTEN__
 	// Stop SDL from calling emscripten_sleep() in functions that are called
 	// indirectly, which does not work with ASYNCIFY_IGNORE_INDIRECT=1. For