[unity] Add cutoff property to Fill shader.

spine-unity/Assets/Spine/Runtime/spine-unity/Modules/Shaders/Spine-Skeleton-Fill.shader

@@ -7,6 +7,7 @@ Shader "Spine/Skeleton Fill" {
 		_FillColor ("FillColor", Color) = (1,1,1,1)
 		_FillPhase ("FillPhase", Range(0, 1)) = 0
 		[NoScaleOffset] _MainTex ("MainTex", 2D) = "white" {}
+		_Cutoff ("Shadow alpha cutoff", Range(0,1)) = 0.1
 		[Toggle(_STRAIGHT_ALPHA_INPUT)] _StraightAlphaInput("Straight Alpha Texture", Int) = 0
 	}
 	SubShader {