Avoid repeating indices while slicing

See: #127 (comment)

Author: lovasoa

src/decoder.rs

@@ -790,8 +790,8 @@ fn compute_image(components: &[Component],
         for y in 0 .. height {
             let destination_idx = y * width;
             let source_idx = y * line_stride;
-            let destination = &mut buffer[destination_idx..destination_idx + width];
-            let source = &data[0][source_idx..source_idx + width];
+            let destination = &mut buffer[destination_idx..][..width];
+            let source = &data[0][source_idx..][..width];
             destination.copy_from_slice(source);
         }