threads: introduce CODEC_CAP_AUTO_THREADS and add it to libx264

Some external codecs have their own code to determine the best number
of threads. This number is not necessary the number of cpu cores.
Thread_count will be only 0 if the codec has CODEC_CAP_AUTO_THREADS.

libavcodec/avcodec.h

@@ -777,6 +777,10 @@ typedef struct RcOverride{
  * Codec supports changed parameters at any point.
  */
 #define CODEC_CAP_PARAM_CHANGE     0x4000
+/**
+ * Codec supports avctx->thread_count == 0 (auto).
+ */
+#define CODEC_CAP_AUTO_THREADS     0x8000
 
 //The following defines may change, don't expect compatibility if you use them.
 #define MB_TYPE_INTRA4x4   0x0001

libavcodec/libx264.c

@@ -592,7 +592,7 @@ AVCodec ff_libx264_encoder = {
     .init           = X264_init,
     .encode         = X264_frame,
     .close          = X264_close,
-    .capabilities   = CODEC_CAP_DELAY,
+    .capabilities   = CODEC_CAP_DELAY | CODEC_CAP_AUTO_THREADS,
     .long_name      = NULL_IF_CONFIG_SMALL("libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
     .priv_class     = &class,
     .defaults       = x264_defaults,

libavcodec/pthread.c

@@ -982,6 +982,9 @@ static void validate_thread_parameters(AVCodecContext *avctx)
     } else if (avctx->codec->capabilities & CODEC_CAP_SLICE_THREADS &&
                avctx->thread_type & FF_THREAD_SLICE) {
         avctx->active_thread_type = FF_THREAD_SLICE;
+    } else if (!(avctx->codec->capabilities & CODEC_CAP_AUTO_THREADS)) {
+        avctx->thread_count       = 1;
+        avctx->active_thread_type = 0;
     }
 }
 

libavcodec/version.h

@@ -22,7 +22,7 @@
 
 #define LIBAVCODEC_VERSION_MAJOR 53
 #define LIBAVCODEC_VERSION_MINOR 32
-#define LIBAVCODEC_VERSION_MICRO  1
+#define LIBAVCODEC_VERSION_MICRO  2
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \
                                                LIBAVCODEC_VERSION_MINOR, \