[HTML5] Fix input not working when buffered.

After input buffering was reworked, input accumulation is now handled
outside of OS, and the JavaScript plaform never implemented that.
Additionally, the JavaScript platform is quite obnoxious about calling
specific APIs outside specific user triggered events.

This commit adds event flushing during the main iteration, and forces it
during keydown/keyup/mousedown/mouseup/touchstart/touchend/touchcanel
events (effectively only accumulating only "move" events).

platform/javascript/os_javascript.cpp

@@ -267,6 +267,10 @@ EM_BOOL OS_JavaScript::keydown_callback(int p_event_type, const EmscriptenKeyboa
 		return false;
 	}
 	os->input->parse_input_event(ev);
+
+	// Make sure to flush all events so we can call restricted APIs inside the event.
+	os->input->flush_buffered_events();
+
 	// Resume audio context after input in case autoplay was denied.
 	os->resume_audio();
 	return true;
@@ -276,13 +280,22 @@ EM_BOOL OS_JavaScript::keypress_callback(int p_event_type, const EmscriptenKeybo
 	OS_JavaScript *os = get_singleton();
 	os->deferred_key_event->set_unicode(p_event->charCode);
 	os->input->parse_input_event(os->deferred_key_event);
+
+	// Make sure to flush all events so we can call restricted APIs inside the event.
+	os->input->flush_buffered_events();
+
 	return true;
 }
 
 EM_BOOL OS_JavaScript::keyup_callback(int p_event_type, const EmscriptenKeyboardEvent *p_event, void *p_user_data) {
+	OS_JavaScript *os = get_singleton();
 	Ref<InputEventKey> ev = setup_key_event(p_event);
 	ev->set_pressed(false);
-	get_singleton()->input->parse_input_event(ev);
+	os->input->parse_input_event(ev);
+
+	// Make sure to flush all events so we can call restricted APIs inside the event.
+	os->input->flush_buffered_events();
+
 	return ev->get_scancode() != KEY_UNKNOWN && ev->get_scancode() != 0;
 }
 
@@ -363,8 +376,13 @@ EM_BOOL OS_JavaScript::mouse_button_callback(int p_event_type, const EmscriptenM
 	ev->set_button_mask(mask);
 
 	os->input->parse_input_event(ev);
+
+	// Make sure to flush all events so we can call restricted APIs inside the event.
+	os->input->flush_buffered_events();
+
 	// Resume audio context after input in case autoplay was denied.
 	os->resume_audio();
+
 	// Prevent multi-click text selection and wheel-click scrolling anchor.
 	// Context menu is prevented through contextmenu event.
 	return true;
@@ -629,6 +647,10 @@ EM_BOOL OS_JavaScript::touch_press_callback(int p_event_type, const EmscriptenTo
 
 		os->input->parse_input_event(ev);
 	}
+
+	// Make sure to flush all events so we can call restricted APIs inside the event.
+	os->input->flush_buffered_events();
+
 	// Resume audio context after input in case autoplay was denied.
 	os->resume_audio();
 	return true;
@@ -954,6 +976,8 @@ bool OS_JavaScript::main_loop_iterate() {
 		godot_js_os_fs_sync(&OS_JavaScript::fs_sync_callback);
 	}
 
+	input->flush_buffered_events();
+
 	if (godot_js_display_gamepad_sample() == OK)
 		process_joypads();