Set scaled img format correctly in vpxdec

Use img->fmt rather than hardcoded I420P to support
profiles 1-3.

Change-Id: I31fc89e545208243e133512d41e2d97995a968ce

vpxdec.c

@@ -889,7 +889,7 @@ int main_loop(int argc, const char **argv_) {
               display_height = display_size[1];
             }
           }
-          scaled_img = vpx_img_alloc(NULL, VPX_IMG_FMT_I420, display_width,
+          scaled_img = vpx_img_alloc(NULL, img->fmt, display_width,
                                      display_height, 16);
           scaled_img->bit_depth = img->bit_depth;
         }