swscale/swscale_unscaled: Try to fix Rgb16ToPlanarRgb16Wrapper() with slices

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e57d99dd4e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/swscale_unscaled.c

@@ -558,6 +558,8 @@ static int Rgb16ToPlanarRgb16Wrapper(SwsContext *c, const uint8_t *src[],
     int bpc = dst_format->comp[0].depth;
     int alpha = src_format->flags & AV_PIX_FMT_FLAG_ALPHA;
     int swap = 0;
+    int i;
+
     if ( HAVE_BIGENDIAN && !(src_format->flags & AV_PIX_FMT_FLAG_BE) ||
         !HAVE_BIGENDIAN &&   src_format->flags & AV_PIX_FMT_FLAG_BE)
         swap++;
@@ -571,6 +573,12 @@ static int Rgb16ToPlanarRgb16Wrapper(SwsContext *c, const uint8_t *src[],
                src_format->name, dst_format->name);
         return srcSliceH;
     }
+
+    for(i=0; i<4; i++) {
+        dst2013[i] += stride2013[i] * srcSliceY / 2;
+        dst1023[i] += stride1023[i] * srcSliceY / 2;
+    }
+
     switch (c->srcFormat) {
     case AV_PIX_FMT_RGB48LE:
     case AV_PIX_FMT_RGB48BE: