Round correctly chroma picture height.

Fix issue 956.

Originally committed as revision 22223 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgconvert.c

@@ -760,8 +760,8 @@ int avpicture_layout(const AVPicture* src, enum PixelFormat pix_fmt, int width,
 
     for (i=0; i<data_planes; i++) {
         if (i == 1) {
-            w = ((width >> desc->log2_chroma_w) * pf->depth + 7) / 8;
-            h = height >> desc->log2_chroma_h;
+            w = (- ((-width) >> desc->log2_chroma_w) * pf->depth + 7) / 8;
+            h = -((-height) >> desc->log2_chroma_h);
             if (pix_fmt == PIX_FMT_NV12 || pix_fmt == PIX_FMT_NV21)
                 w <<= 1;
         } else if (i == 3) {