improve color averaging

src/main/java/eu/jonahbauer/raytracing/render/texture/Color.java

@@ -44,10 +44,11 @@ public record Color(double r, double g, double b) implements Texture {
     }
 
     public static @NotNull Color average(@NotNull Color current, @NotNull Color next, int index) {
+        var factor = 1d / index;
         return new Color(
-                current.r() + (next.r() - current.r()) / index,
-                current.g() + (next.g() - current.g()) / index,
-                current.b() + (next.b() - current.b()) / index
+                Math.fma(factor, next.r() - current.r(), current.r()),
+                Math.fma(factor, next.g() - current.g(), current.g()),
+                Math.fma(factor, next.b() - current.b(), current.b())
         );
     }