Add project setting for agile input event flushing

If enabled, key/touch/joystick events will be flushed just before every idle and physics frame.

Enabling this can greatly improve the responsiveness to input, specially in devices that need to run multiple physics frames per each idle frame, because of not being powerful enough to run at the target frame rate.

This will only work for platforms using input buffering (regardless event accumulation). Currenly, only Android does so, but could be implemented for iOS in an upcoming PR.

doc/classes/ProjectSettings.xml

@@ -717,6 +717,12 @@
 			Default [InputEventAction] to move up in the UI.
 			[b]Note:[/b] Default [code]ui_*[/code] actions cannot be removed as they are necessary for the internal logic of several [Control]s. The events assigned to the action can however be modified.
 		</member>
+		<member name="input_devices/buffering/agile_event_flushing" type="bool" setter="" getter="" default="false">
+			If [code]true[/code], key/touch/joystick events will be flushed just before every idle and physics frame.
+			If [code]false[/code], such events will be flushed only once per process frame, between iterations of the engine.
+			Enabling this can greatly improve the responsiveness to input, specially in devices that need to run multiple physics frames per visible (process) frame, because they can't run at the target frame rate.
+			[b]Note:[/b] Currently implemented only on Android.
+		</member>
 		<member name="input_devices/pen_tablet/driver" type="String" setter="" getter="">
 			Specifies the tablet driver to use. If left empty, the default driver will be used.
 		</member>

main/main.cpp

@@ -1735,6 +1735,8 @@ Error Main::setup2(Thread::ID p_main_tid_override) {
 
 	Input *id = Input::get_singleton();
 	if (id) {
+		agile_input_event_flushing = GLOBAL_DEF("input_devices/buffering/agile_event_flushing", false);
+
 		if (bool(GLOBAL_DEF("input_devices/pointing/emulate_touch_from_mouse", false)) &&
 				!(editor || project_manager)) {
 			bool found_touchscreen = false;
@@ -2442,6 +2444,7 @@ uint32_t Main::frames = 0;
 uint32_t Main::frame = 0;
 bool Main::force_redraw_requested = false;
 int Main::iterating = 0;
+bool Main::agile_input_event_flushing = false;
 
 bool Main::is_iterating() {
 	return iterating > 0;
@@ -2491,9 +2494,13 @@ bool Main::iteration() {
 
 	bool exit = false;
 
-	Engine::get_singleton()->_in_physics = true;
-
 	for (int iters = 0; iters < advance.physics_steps; ++iters) {
+		if (Input::get_singleton()->is_using_input_buffering() && agile_input_event_flushing) {
+			Input::get_singleton()->flush_buffered_events();
+		}
+
+		Engine::get_singleton()->_in_physics = true;
+
 		uint64_t physics_begin = OS::get_singleton()->get_ticks_usec();
 
 		PhysicsServer3D::get_singleton()->sync();
@@ -2522,9 +2529,13 @@ bool Main::iteration() {
 		physics_process_ticks = MAX(physics_process_ticks, OS::get_singleton()->get_ticks_usec() - physics_begin); // keep the largest one for reference
 		physics_process_max = MAX(OS::get_singleton()->get_ticks_usec() - physics_begin, physics_process_max);
 		Engine::get_singleton()->_physics_frames++;
+
+		Engine::get_singleton()->_in_physics = false;
 	}
 
-	Engine::get_singleton()->_in_physics = false;
+	if (Input::get_singleton()->is_using_input_buffering() && agile_input_event_flushing) {
+		Input::get_singleton()->flush_buffered_events();
+	}
 
 	uint64_t process_begin = OS::get_singleton()->get_ticks_usec();
 
@@ -2588,7 +2599,7 @@ bool Main::iteration() {
 	iterating--;
 
 	// Needed for OSs using input buffering regardless accumulation (like Android)
-	if (Input::get_singleton()->is_using_input_buffering()) {
+	if (Input::get_singleton()->is_using_input_buffering() && !agile_input_event_flushing) {
 		Input::get_singleton()->flush_buffered_events();
 	}
 

main/main.h

@@ -42,6 +42,7 @@ class Main {
 	static uint32_t frame;
 	static bool force_redraw_requested;
 	static int iterating;
+	static bool agile_input_event_flushing;
 
 public:
 	static bool is_project_manager();