vp9: use superframe split BSF

webm usually has invisible superframes merged with normal frames.
(vpxenc muxes them in this form, which is evidence enough that this is
the standard webm packet format. It's rather unclear whether ffmpeg is
even allowed to remux them with split packets.)

The vp9 decoder needs them to be in separate packets for multithreading
to work. Add the BSF to the decoder, so the conversion happens
automatically.

This contains the important part of fa1749dd, which
was apparently skipped in commit d417e95a. This restores Libav API
compatibility.

libavcodec/version.h

@@ -29,7 +29,7 @@
 
 #define LIBAVCODEC_VERSION_MAJOR  58
 #define LIBAVCODEC_VERSION_MINOR   6
-#define LIBAVCODEC_VERSION_MICRO 101
+#define LIBAVCODEC_VERSION_MICRO 102
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \
                                                LIBAVCODEC_VERSION_MINOR, \

libavcodec/vp9.c

@@ -1795,6 +1795,7 @@ AVCodec ff_vp9_decoder = {
     .init_thread_copy      = ONLY_IF_THREADS_ENABLED(vp9_decode_init_thread_copy),
     .update_thread_context = ONLY_IF_THREADS_ENABLED(vp9_decode_update_thread_context),
     .profiles              = NULL_IF_CONFIG_SMALL(ff_vp9_profiles),
+    .bsfs                  = "vp9_superframe_split",
     .hw_configs            = (const AVCodecHWConfigInternal*[]) {
 #if CONFIG_VP9_DXVA2_HWACCEL
                                HWACCEL_DXVA2(vp9),