Shader "MaskedTexture"
{
   Properties
   {
      _MainTex ("Base (RGB)", 2D) = "white" {}
      _Mask ("Culling Mask", 2D) = "white" {}
   }
   SubShader
   {
      Tags {"Queue"="Transparent"}
      Lighting Off
      ZWrite Off
      Blend SrcAlpha OneMinusSrcAlpha
      Pass
      {
         SetTexture [_Mask] {combine texture}
         SetTexture [_MainTex] {combine texture, previous}
      }
   }
}