Render to texture isn't setting the texture

I have a shader that takes a texture and adds a transparent overlay. It is blue in the image below:



Shader "Custom/Overlay"
{
Properties
{
_MainTex ("Texture", Rect) = "white" {}
_Color ("Color", Color) = (0.0, 0.0, 0.0, 0.0)
}
SubShader
{
Tags { "Queue" = "Overlay" } // Render after everything else

Pass
{
Blend SrcAlpha OneMinusSrcAlpha // Alpha blending
ZWrite Off
ZTest Always // Deactivate depth test

CGPROGRAM

#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"

sampler2D _MainTex;
uniform float4 _Color;

struct VertexShaderInput
{
float4 Position : POSITION0;
float2 TextureCoordinates : TEXCOORD0;
};

struct VertexShaderOutput
{
float4 Position : POSITION0;
float2 TextureCoordinates : TEXCOORD0;
};

VertexShaderOutput vert(VertexShaderInput input)
{
VertexShaderOutput output;

output.Position = input.Position;
output.Position.w = 1.0;

output.TextureCoordinates = input.TextureCoordinates;

return output;
}

float4 frag(VertexShaderOutput input) : COLOR
{
float4 backgroundCol = tex2D(_MainTex, input.TextureCoordinates);

return float4(lerp(backgroundCol, _Color, _Color.a).rgb, 1.0);
}

ENDCG
}
}
}

If I change the Target Texture then the target texture never gets written to with the blue image that is on the screen.

Why isn't the blue tinted image being written to the Target Texture when it displays just fine on the screen when there is no Target Texture set?

 

After some digging I believe I will need to handle the unsetting of render targets in OnPostRender()

http://docs.unity3d.com/ScriptReference/MonoBehaviour.OnPostRender.html

This is probably what is causing this issue.