[unity] Fix Tint Black shader alpha.

spine-unity/Assets/spine-unity/Shaders/Spine-Skeleton-TintBlack.shader

@@ -61,7 +61,7 @@ Shader "Spine/Skeleton Tint Black" {
 
 			float4 frag (VertexOutput i) : COLOR {
 				float4 texColor = tex2D(_MainTex, i.uv);
-				return (texColor * i.vertexColor) + float4(((1-texColor.rgb) * texColor.a * (_Black.rgb + float3(i.uv1.r, i.uv1.g, i.uv2.r))), 1);
+				return (texColor * i.vertexColor) + float4(((1-texColor.rgb) * texColor.a * (_Black.rgb + float3(i.uv1.r, i.uv1.g, i.uv2.r))), 0);
 			}
 			ENDCG
 		}