mention avcodec_align_dimensions()

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@3136 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/avcodec.h

@@ -897,7 +897,8 @@ typedef struct AVCodecContext {
     /**
      * called at the beginning of each frame to get a buffer for it.
      * if pic.reference is set then the frame will be read later by lavc
-     * width and height should be rounded up to the next multiple of 16
+     * avcodec_align_dimensions() should be used to find the required width and
+     * height, as they normally need to be rounded up to the next multiple of 16
      * - encoding: unused
      * - decoding: set by lavc, user can override
      */