game: add fps overlay

game/overlay_fps.gd

@@ -0,0 +1,17 @@
+extends CanvasLayer
+
+@onready var text = $RichTextLabel
+@onready var timer = $Timer
+
+func _ready() -> void:
+	visible = false
+
+func _on_timer_timeout() -> void:
+	text.clear()
+	text.add_text("FPS: %s" % Engine.get_frames_per_second())
+
+func _on_visibility_changed() -> void:
+	if visible:
+		timer.start()
+	else:
+		timer.stop()