lavc/videotoolbox: call VTRegisterSupplementalVideoDecoderIfAvailable

This is required for VP9 to work.

libavcodec/videotoolbox.c

@@ -32,6 +32,7 @@
 #include "h264dec.h"
 #include "hevcdec.h"
 #include "mpegvideo.h"
+#include <Availability.h>
 #include <TargetConditionals.h>
 
 #ifndef kVTVideoDecoderSpecification_RequireHardwareAcceleratedVideoDecoder
@@ -866,6 +867,12 @@ static int videotoolbox_start(AVCodecContext *avctx)
         break;
     }
 
+#ifdef __MAC_10_11
+    if (__builtin_available(macOS 10.11, *)) {
+        VTRegisterSupplementalVideoDecoderIfAvailable(videotoolbox->cm_codec_type);
+    }
+#endif
+
     decoder_spec = videotoolbox_decoder_config_create(videotoolbox->cm_codec_type, avctx);
 
     if (!decoder_spec) {