[normative]Set img's monochrome value correctly

BUG=aomedia:2051

Change-Id: I666440d7a273106572ff2afdb895b9255cf363b9
diff --git a/apps/aomdec.c b/apps/aomdec.c
index 2d8a962..1e34eaa 100644
--- a/apps/aomdec.c
+++ b/apps/aomdec.c
@@ -937,6 +937,9 @@
               aom_img_free(img_shifted);
               img_shifted = NULL;
             }
+            if (img_shifted) {
+              img_shifted->monochrome = img->monochrome;
+            }
             if (!img_shifted) {
               img_shifted =
                   aom_img_alloc(NULL, shifted_fmt, img->d_w, img->d_h, 16);