document intermediate

src/scene.rs

@@ -38,6 +38,9 @@ impl Scene {
     }
 }
 
+/// scene name is derived from the filename,
+/// and None values are represented as "" in the toml
+/// so it's Vec<String> instead of Vec<Option<String>>
 #[derive(Serialize, Deserialize)]
 struct IntermediateScene {
     background: Vec<String>,