PlaceholderBehavior's sprite doe snot work with Light Only mode

[leogeier]

To reproduce:

1. Create a PlaceholderBehavior using a sprite
2. Add a CanvasItemMaterial to it, set Light Mode to Light Only
3. Add a PointLight to the scene

The PlaceholderBehavior will be visible even if it is outside the PointLight.

A simple way to fix this is to add the following line to the sprite outline shader (res://addons/pronto/shader/outline.gdshader): render_mode blend_mix,light_only;. However, this would necessitate multiple versions of the shader.