Move classes to own files.

src/main/kotlin/Scene.kt

@@ -0,0 +1,21 @@
+data class Scene(val things: List<Thing>, val light: PointLight): Thing {
+    override fun intersects(ray: Ray): Hit? {
+        var closest: Hit? = null
+        for (thing in things) {
+            val hit = thing.intersects(ray)
+            if (hit != null && (closest == null || hit.distance < closest.distance)) {
+                closest = hit
+            }
+        }
+
+        return closest
+    }
+
+    fun colorForRay(ray: Ray): MaterialColor? {
+        val hit = intersects(ray)
+        if (hit != null) {
+            return hit.material.shade(ray, hit, this)
+        }
+        return null
+    }
+}