lavc: Move brd_scale to codec private options

This option is only used by mpegvideoenc,
It is a very codec-specific option, so deprecate the global variant.
Set proper limits to the maximum allowed values.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

libavcodec/avcodec.h

@@ -1958,12 +1958,11 @@ typedef struct AVCodecContext {
      */
     int bidir_refine;
 
-    /**
-     *
-     * - encoding: Set by user.
-     * - decoding: unused
-     */
+#if FF_API_PRIVATE_OPT
+    /** @deprecated use encoder private options instead */
+    attribute_deprecated
     int brd_scale;
+#endif
 
     /**
      * minimum GOP size