Use gray_bench_image in filter benchmarks (#657)

src/filter/mod.rs

@@ -1020,9 +1020,7 @@ mod benches {
         ($name:ident, $kernel_size:expr, $image_size:expr, $function_name:ident) => {
             #[bench]
             fn $name(b: &mut Bencher) {
-                let image =
-                    GrayImage::from_fn($image_size, $image_size, |x, y| Luma([(x + y % 3) as u8]));
-
+                let image = gray_bench_image($image_size, $image_size);
                 let kernel: Vec<i32> = (0..$kernel_size * $kernel_size).collect();
                 let kernel = Kernel::new(&kernel, $kernel_size, $kernel_size);
                 b.iter(|| {
@@ -1037,7 +1035,6 @@ mod benches {
             #[bench]
             fn $name(b: &mut Bencher) {
                 let image = rgb_bench_image($image_size, $image_size);
-
                 let kernel: Vec<i32> = (0..$kernel_size * $kernel_size).collect();
                 let kernel = Kernel::new(&kernel, $kernel_size, $kernel_size);
                 b.iter(|| {