Deprecate obsolete XvMC hardware decoding support

XvMC has long ago been superseded by newer acceleration APIs, such as
VDPAU, and few downstreams still support it. Furthermore XvMC is not
implemented within the hwaccel framework, but requires its own specific
code in the MPEG-1/2 decoder, which is a maintenance burden.

libavcodec/avcodec.h

@@ -103,7 +103,9 @@ enum AVCodecID {
     /* video codecs */
     AV_CODEC_ID_MPEG1VIDEO,
     AV_CODEC_ID_MPEG2VIDEO, ///< preferred ID for MPEG-1/2 video decoding
+#if FF_API_XVMC
     AV_CODEC_ID_MPEG2VIDEO_XVMC,
+#endif /* FF_API_XVMC */
     AV_CODEC_ID_H261,
     AV_CODEC_ID_H263,
     AV_CODEC_ID_RV10,
@@ -691,8 +693,10 @@ typedef struct RcOverride{
  */
 #define CODEC_CAP_DR1             0x0002
 #define CODEC_CAP_TRUNCATED       0x0008
+#if FF_API_XVMC
 /* Codec can export data for HW decoding (XvMC). */
 #define CODEC_CAP_HWACCEL         0x0010
+#endif /* FF_API_XVMC */
 /**
  * Encoder or decoder requires flushing with NULL input at the end in order to
  * give the complete and correct output.
@@ -1526,12 +1530,15 @@ typedef struct AVCodecContext {
 #define SLICE_FLAG_ALLOW_FIELD    0x0002 ///< allow draw_horiz_band() with field slices (MPEG2 field pics)
 #define SLICE_FLAG_ALLOW_PLANE    0x0004 ///< allow draw_horiz_band() with 1 component at a time (SVQ1)
 
+#if FF_API_XVMC
     /**
      * XVideo Motion Acceleration
      * - encoding: forbidden
      * - decoding: set by decoder
+     * @deprecated XvMC support is slated for removal.
      */
-    int xvmc_acceleration;
+    attribute_deprecated int xvmc_acceleration;
+#endif /* FF_API_XVMC */
 
     /**
      * macroblock decision mode