more cave stuff

scripts/fix_parallax_background.gd

@@ -0,0 +1,20 @@
+extends ParallaxBackground
+
+func revert_offset(layer: ParallaxLayer) -> void:
+	# Cancel out layer's offset. The layer's position already has 
+	# its motion_scale applied.
+	var ofs := scroll_offset - layer.position
+	if not scroll_ignore_camera_zoom:
+		# When attention is given to the camera's zoom, we need to account for it.
+		# We can use viewport's canvas transform scale to which the camera has
+		# already applied its zoom.
+		var canvas_scale = get_viewport().canvas_transform.get_scale()
+		# This is taken from godot source: parallax_background.cpp
+		# I don't know why it works.
+		ofs /= canvas_scale.dot(Vector2(0.5, 0.5))
+	layer.motion_offset = ofs
+
+func _ready() -> void:
+	for layer in get_children():
+		if layer is ParallaxLayer:
+			revert_offset(layer)