avcodec/utils: Document 32 min for h264 width

Suggested-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/utils.c

@@ -432,6 +432,11 @@ void avcodec_align_dimensions2(AVCodecContext *s, int *width, int *height,
         // some of the optimized chroma MC reads one line too much
         // which is also done in mpeg decoders with lowres > 0
         *height += 2;
+
+        // H.264 uses edge emulation for out of frame motion vectors, for this
+        // it requires a temporary area large enough to hold a 21x21 block,
+        // increasing witdth ensure that the temporary area is large enough,
+        // the next rounded up width is 32
         *width = FFMAX(*width, 32);
     }