Add input buffering framework

Input buffering is implicitly used by event accumulation, but this commit makes it more generic so it can be enabled for other uses.

For desktop OSs it's currently not feasible given main and UI threads are the same).

main/main.cpp

@@ -2587,6 +2587,11 @@ bool Main::iteration() {
 
 	iterating--;
 
+	// Needed for OSs using input buffering regardless accumulation (like Android)
+	if (Input::get_singleton()->is_using_input_buffering()) {
+		Input::get_singleton()->flush_buffered_events();
+	}
+
 	if (fixed_fps != -1) {
 		return exit;
 	}