Fix swapped width and height in compute_image

See https://github.com/image-rs/jpeg-decoder/pull/117/files/67dc09e3712832d6970c63a0fb4c94ccd619e2e2#r355657982

Author: lovasoa

src/decoder.rs

@@ -787,8 +787,8 @@ fn compute_image(components: &[Component],
         let mut buffer = vec![0u8; width * height];
         let line_stride = component.block_size.width as usize * component.dct_scale;
 
-        for y in 0 .. width {
-            for x in 0 .. height {
+        for y in 0 .. height {
+            for x in 0 .. width {
                 buffer[y * width + x] = data[0][y * line_stride + x];
             }
         }