lavc: Move b_frame_strategy and b_sensitivity to codec private options

The b_frame_strategy option is only used by mpegvideoenc, qsv, x264, and
xavs, while b_sensitivity is only used by mpegvideoenc.

These are very codec-specific options, so deprecate the global variants.
Set proper limits to the maximum allowed values.

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

libavcodec/avcodec.h

@@ -1594,7 +1594,11 @@ typedef struct AVCodecContext {
 #define FF_RC_STRATEGY_XVID 1
 #endif
 
+#if FF_API_PRIVATE_OPT
+    /** @deprecated use encoder private options instead */
+    attribute_deprecated
     int b_frame_strategy;
+#endif
 
     /**
      * qscale offset between IP and B-frames
@@ -1999,12 +2003,11 @@ typedef struct AVCodecContext {
      */
     int mv0_threshold;
 
-    /**
-     * Adjust sensitivity of b_frame_strategy 1.
-     * - encoding: Set by user.
-     * - decoding: unused
-     */
+#if FF_API_PRIVATE_OPT
+    /** @deprecated use encoder private options instead */
+    attribute_deprecated
     int b_sensitivity;
+#endif
 
     /**
      * Chromaticity coordinates of the source primaries.