Unity3d skybox rotation shader

Shader "RenderFX/Skybox-CustomRotation"
{
	Properties
	{
		_Tint ("Tint Color", Color) = (.5, .5, .5, .5)
		_Tex ("Cubemap", Cube) = "white" {}
	}

	SubShader
	{
		Tags { "Queue"="Background" "RenderType"="Background" }
		Cull Off ZWrite Off Fog { Mode Off }

		Pass
		{

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma fragmentoption ARB_precision_hint_fastest

			#include "UnityCG.cginc"

			samplerCUBE _Tex;
			fixed4 _Tint;
			float4x4 _Rotation;

			struct appdata_t
			{
				float4 vertex : POSITION;
				float3 texcoord : TEXCOORD0;
			};

			struct v2f
			{
				float4 vertex : POSITION;
				float3 texcoord : TEXCOORD0;
			};

			v2f vert(appdata_t v)
			{
			    v2f o;
			    o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
			    o.texcoord = mul((float3x3)_Rotation, v.texcoord);
			    return o;
			}

			fixed4 frag (v2f i) : COLOR
			{
				fixed4 tex = texCUBE (_Tex, i.texcoord);
				fixed4 col;
				col.rgb = tex.rgb + _Tint.rgb - unity_ColorSpaceGrey;
				col.a = tex.a * _Tint.a;
				return col;
			}
			ENDCG
		}
	}
}