Add light as associated type for scene and get something to draw with pbrt materials.

ray-tracing-pbrt-scene/examples/pbrt-test.rs

@@ -1,4 +1,5 @@
 use clap::Parser;
+use rand::rngs::SmallRng;
 use ray_tracing_pbrt_scene::parse_pbrt_v4;
 use std::path::PathBuf;
 
@@ -10,7 +11,7 @@ struct Args {
 fn main() -> Result<(), miette::Error> {
     let args = Args::parse();
 
-    let _t = parse_pbrt_v4(args.filename)?;
+    let _t = parse_pbrt_v4::<SmallRng>(args.filename)?;
 
     // dbg!(t);