lavc: Move prediction_method to codec private options

This options is only used by huffyuv, ffvhuv, jpegls, mjpeg,
mpegvideoenc, png, utvideo.
It is a very codec-specific options, so deprecate the global variant.
Set proper limits to the maximum allowed values, and update utvideoenc
tests to use the new option name.

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

libavcodec/avcodec.h

@@ -1678,15 +1678,15 @@ typedef struct AVCodecContext {
      * - decoding: Set by user (or 0).
      */
     int slice_count;
-    /**
-     * prediction method (needed for huffyuv)
-     * - encoding: Set by user.
-     * - decoding: unused
-     */
+
+#if FF_API_PRIVATE_OPT
+    /** @deprecated use encoder private options instead */
+    attribute_deprecated
      int prediction_method;
 #define FF_PRED_LEFT   0
 #define FF_PRED_PLANE  1
 #define FF_PRED_MEDIAN 2
+#endif
 
     /**
      * slice offsets in the frame in bytes