avcodec/mjpegdec: Avoid copying data when flipping image

Basically reverts af15c17daa.
Flipping a picture by modifying the pointers is so common
that even users of direct rendering should take it into account.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/mjpegdec.c

@@ -2770,28 +2770,18 @@ the_end:
         }
     }
     if (s->flipped && !s->rgb) {
-        int j;
         ret = av_pix_fmt_get_chroma_sub_sample(s->avctx->pix_fmt, &hshift, &vshift);
         if (ret)
             return ret;
 
-        av_assert0(s->nb_components == av_pix_fmt_count_planes(s->picture_ptr->format));
+        av_assert0(s->nb_components == av_pix_fmt_count_planes(frame->format));
         for (index=0; index<s->nb_components; index++) {
-            uint8_t *dst = s->picture_ptr->data[index];
-            int w = s->picture_ptr->width;
-            int h = s->picture_ptr->height;
-            if(index && index<3){
-                w = AV_CEIL_RSHIFT(w, hshift);
+            int h = frame->height;
+            if (index && index < 3)
                 h = AV_CEIL_RSHIFT(h, vshift);
-            }
-            if(dst){
-                uint8_t *dst2 = dst + s->picture_ptr->linesize[index]*(h-1);
-                for (i=0; i<h/2; i++) {
-                    for (j=0; j<w; j++)
-                        FFSWAP(int, dst[j], dst2[j]);
-                    dst  += s->picture_ptr->linesize[index];
-                    dst2 -= s->picture_ptr->linesize[index];
-                }
+            if (frame->data[index]) {
+                frame->data[index]     += (h - 1) * frame->linesize[index];
+                frame->linesize[index] *= -1;
             }
         }
     }