non-VR portal rendering

Assets/Escape Room Engine/Portal/PortalShader.shader

@@ -0,0 +1,55 @@
+Shader "Escape Room Engine/Portal"
+{
+    Properties
+    {
+        _MainTex("Portal Texture", 2D) = "" {}
+    }
+
+    SubShader
+    {
+        Tags { "RenderType" = "Opaque" "RenderPipeline" = "UniversalPipeline" "Queue" = "Geometry" }
+
+        Pass
+        {
+            // stencil the portal surface to cut it out from the portal frame
+            Stencil
+            {
+                Ref 1
+                Pass replace
+            }
+            
+            HLSLPROGRAM
+            #pragma vertex vert
+            #pragma fragment frag
+
+            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
+
+            uniform sampler2D _MainTex;
+
+            struct Attributes
+            {
+                float4 vertex : POSITION;
+            };
+
+            struct Varyings
+            {
+                float4 vertex : SV_POSITION;
+                float4 screen : TEXCOORD0;
+            };
+
+            Varyings vert(Attributes IN)
+            {
+                Varyings OUT;
+                OUT.vertex = TransformObjectToHClip(IN.vertex.xyz);
+                OUT.screen = ComputeScreenPos(OUT.vertex);
+                return OUT;
+            }
+
+            half4 frag(Varyings IN) : SV_Target
+            {
+                return tex2D(_MainTex, IN.screen.xy / IN.screen.w);
+            }
+            ENDHLSL
+        }
+    }
+}