avcodec/h263dec: Avoid copying data when flipping image

Also remove an unchecked av_frame_make_writable().
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/h263dec.c

@@ -703,17 +703,11 @@ frame_end:
     if (s->last_picture_ptr || s->low_delay) {
         if (   pict->format == AV_PIX_FMT_YUV420P
             && (s->codec_tag == AV_RL32("GEOV") || s->codec_tag == AV_RL32("GEOX"))) {
-            int x, y, p;
-            av_frame_make_writable(pict);
-            for (p=0; p<3; p++) {
-                int w = AV_CEIL_RSHIFT(pict-> width, !!p);
+            for (int p = 0; p < 3; p++) {
                 int h = AV_CEIL_RSHIFT(pict->height, !!p);
-                int linesize = pict->linesize[p];
-                for (y=0; y<(h>>1); y++)
-                    for (x=0; x<w; x++)
-                        FFSWAP(int,
-                               pict->data[p][x + y*linesize],
-                               pict->data[p][x + (h-1-y)*linesize]);
+
+                pict->data[p]     += (h - 1) * pict->linesize[p];
+                pict->linesize[p] *= -1;
             }
         }
         *got_frame = 1;