Added a few test images

The blank test image was generated with
    convert -size 800x280 -sampling-factor 2x2 xc:white tests/reftest/images/blank_800x280.jpg

This creates a grayscale image with a sampling factor of 2 and a height that is a multiple of 8 but not 16,
so that it triggers #148.

Author: lovasoa

benches/decoding_benchmark.rs

@@ -26,6 +26,10 @@ fn main() {
         read_image(include_bytes!("tower_progressive.jpg"))
     }));
 
+    c.bench_function("decode a 512x512 grayscale JPEG", |b| b.iter(|| {
+        read_image(include_bytes!("tower_grayscale.jpg"))
+    }));
+
     c.bench_function("extract metadata from an image", |b| b.iter(|| {
         read_metadata(include_bytes!("tower.jpg"))
     }));