optimisation: only render visible portals

Assets/Escape Room Engine/Portal/Portal.cs

@@ -60,6 +60,12 @@ namespace Escape_Room_Engine.Portal
 
         private void Render(ScriptableRenderContext scriptableRenderContext, Camera camera)
         {
+            // check whether the portal plane is visible from the player camera
+            var frustumPlanes = GeometryUtility.CalculateFrustumPlanes(_playerCamera.camera);
+            if (!GeometryUtility.TestPlanesAABB(frustumPlanes, other.portalQuad.bounds))
+                // don't render this portal if it is not visible
+                return;
+            
             var t = transform;
 
             foreach (var eye in Eyes)