Make a standard Unity Quad full-screen, purely in the shader

FullScreenQuad.shader

//
// 1. Add a Quad in Unity
// 2. Parent the quad under camera, to prevent frustum culling.
// 3. Attach this shader.
//
Shader "Quad/Fullscreen"
{
	Properties
	{
	}

	SubShader
	{
		Cull Off ZWrite Off

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv     : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				float4 vertex : SV_POSITION;
			};

			v2f vert(appdata v)
			{
				v2f o;
				o.vertex = v.vertex;
				o.vertex.xy *= 2;
				o.uv = v.uv;
				return o;
			}

			fixed4 frag(v2f i) : SV_Target
			{
				return fixed4(i.uv, 0, 1);
			}
			ENDCG
		}
	}
}

Hi I'm trying to use this shader but having trouble merging this with the shader I already have. This is my shader:
`Shader "alphaMaskShader"
{
Properties{
_MainTex("Base (RGB)", 2D) = "white" {}
_Alpha("Alpha (A)", 2D) = "white" {}
}
SubShader{
Tags{ "RenderType" = "Transparent" "Queue" = "Overlay" }

	ZWrite Off
	ZTest Off

	Blend SrcAlpha OneMinusSrcAlpha
	//ColorMask RGB

	Pass{
	    SetTexture[_MainTex]{
	        Combine texture
            }
	    SetTexture[_Alpha]{
	        Combine previous, texture
            }
        }
}

}`

I tried inserting your gist right after the pass but it isn't rendering anything. Are these just incompatible?