avcodec/avcodec.h: move debug_mv to the end of the struct with the next ABI bump

This maintains ABI compatibility
also add a note that debug_mv must be accessed through AVOptions (for ABI)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -2469,6 +2469,7 @@ typedef struct AVCodecContext {
 #if FF_API_DEBUG_MV
     /**
      * debug
+     * Code outside libavcodec should access this field using AVOptions
      * - encoding: Set by user.
      * - decoding: Set by user.
      */
@@ -2907,6 +2908,19 @@ typedef struct AVCodecContext {
      * - encoding: set by libavcodec
      */
     int seek_preroll;
+
+#if !FF_API_DEBUG_MV
+    /**
+     * debug motion vectors
+     * Code outside libavcodec should access this field using AVOptions
+     * - encoding: Set by user.
+     * - decoding: Set by user.
+     */
+    int debug_mv;
+#define FF_DEBUG_VIS_MV_P_FOR  0x00000001 //visualize forward predicted MVs of P frames
+#define FF_DEBUG_VIS_MV_B_FOR  0x00000002 //visualize forward predicted MVs of B frames
+#define FF_DEBUG_VIS_MV_B_BACK 0x00000004 //visualize backward predicted MVs of B frames
+#endif
 } AVCodecContext;
 
 AVRational av_codec_get_pkt_timebase         (const AVCodecContext *avctx);