A unity sprite outline solution. unity verson:5.5.2f1
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1 Outline is a seperate gameobject, the order in the layer can be set.
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2 Use outline shader to render the sprite's texture only once (when it appear in the scene). Use setPropertyBlock to change the sprite textures.
Sprite outline shader modifies from http://nielson.io/2016/04/2d-sprite-outlines-in-unity/ by Ryan Nielson.