Portal rendering 1/2 done

Assets/Shaders/ScreenCutoutShader.shader

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+// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'
+
+Shader "Unlit/ScreenCutoutShader"
+{
+	Properties
+	{
+		_MainTex ("Texture", 2D) = "white" {}
+	}
+	SubShader
+	{
+		Tags{ "Queue" = "Transparent" "IgnoreProjector" = "True" "RenderType" = "Transparent" }
+		Lighting Off
+		Cull Back
+		ZWrite On
+		ZTest Less
+		
+		Fog{ Mode 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;
+				float4 screenPos : TEXCOORD1;
+			};
+
+			v2f vert (appdata v)
+			{
+				v2f o;
+				o.vertex = UnityObjectToClipPos(v.vertex);
+				o.screenPos = ComputeScreenPos(o.vertex);
+				return o;
+			}
+			
+			sampler2D _MainTex;
+
+			fixed4 frag (v2f i) : SV_Target
+			{
+				i.screenPos /= i.screenPos.w;
+				fixed4 col = tex2D(_MainTex, float2(i.screenPos.x, i.screenPos.y));
+				
+				return col;
+			}
+			ENDCG
+		}
+	}
+}