Optimize vertical box filter to iterate over rows for cache friendliness.

src/main.rs

@@ -2,7 +2,4 @@ mod blur;
 mod grid;
 mod model;
 
-fn main() {
-    let boxes = blur::boxes_for_gaussian::<3>(2.5);
-    println!("boxes: {:?}", boxes);
-}
+fn main() {}