title: Sprite disappear shader
body: <p>i am working on Unity 2D platform game.
  i am using a shader i found <a href="http://www.shaderslab.com/index.php?post/Dissolve-sprite" rel="nofollow noreferrer">here</a>, but when i flip my sprite (scale -1),
  the sprite disappeared, i have little understand in shader coding,
  And i didn't found any help on Google, can any one help fixing the shader?.
  here is the shader:</p>
  
  <pre><code>Properties {
      [PerRendererData] _MainTex ("Main texture", 2D) = "white" {}
      _DissolveTex ("Dissolution texture", 2D) = "gray" {}
      _Threshold ("Threshold", Range(0., 1.01)) = 0.
  }
  
  SubShader {
  
      Tags { "Queue"="Transparent" }
      Blend SrcAlpha OneMinusSrcAlpha
  
      Pass {
  
          CGPROGRAM
          #pragma vertex vert
          #pragma fragment frag
  
          #include "UnityCG.cginc"
  
          struct v2f {
              float4 pos : SV_POSITION;
              float2 uv : TEXCOORD0;
          };
  
          sampler2D _MainTex;
  
          v2f vert(appdata_base v) {
              v2f o;
              o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
              o.uv = v.texcoord;
              return o;
          }
  
          sampler2D _DissolveTex;
          float _Threshold;
  
          fixed4 frag(v2f i) : SV_Target {
              float4 c = tex2D(_MainTex, i.uv);
              float val = tex2D(_DissolveTex, i.uv).r;
  
              c.a *= step(_Threshold, val);
              return c;
          }
          ENDCG
      }
  }
  </code></pre>