avcodec/videotoolbox: fix decoding of some h264 bitstreams

This affects Annex B streams (such as demuxed from .ts and others). It
also handles the format change in reinit-large_420_8-to-small_420_8.h264
correctly.

Instead of passing through the extradata, create it on the fly it from
the currently active SPS and PPS. Since reconstructing the PPS and SPS
NALs would be very complicated and verbose, we use the NALs as they
originally appeared in the bitstream.

The code for writing the extradata is somewhat derived from
libavformat/avc.c, but it's small and different enough that sharing it
is not really worth it.

libavcodec/videotoolbox.c

@@ -77,28 +77,40 @@ int ff_videotoolbox_alloc_frame(AVCodecContext *avctx, AVFrame *frame)
     return 0;
 }
 
+#define AV_W8(p, v) *(p) = (v)
+
 CFDataRef ff_videotoolbox_avcc_extradata_create(AVCodecContext *avctx)
 {
+    H264Context *h     = avctx->priv_data;
     CFDataRef data = NULL;
-
-    /* Each VCL NAL in the bitstream sent to the decoder
-     * is preceded by a 4 bytes length header.
-     * Change the avcC atom header if needed, to signal headers of 4 bytes. */
-    if (avctx->extradata_size >= 4 && (avctx->extradata[4] & 0x03) != 0x03) {
-        uint8_t *rw_extradata = av_memdup(avctx->extradata, avctx->extradata_size);
-
-        if (!rw_extradata)
+    uint8_t *p;
+    int vt_extradata_size = 6 + 3 + h->sps.data_size + 4 + h->pps.data_size;
+    uint8_t *vt_extradata = av_malloc(vt_extradata_size);
+    if (!vt_extradata)
         return NULL;
 
-        rw_extradata[4] |= 0x03;
-
-        data = CFDataCreate(kCFAllocatorDefault, rw_extradata, avctx->extradata_size);
-
-        av_freep(&rw_extradata);
-    } else {
-        data = CFDataCreate(kCFAllocatorDefault, avctx->extradata, avctx->extradata_size);
-    }
-
+    p = vt_extradata;
+
+    AV_W8(p + 0, 1); /* version */
+    AV_W8(p + 1, h->sps.data[0]); /* profile */
+    AV_W8(p + 2, h->sps.data[1]); /* profile compat */
+    AV_W8(p + 3, h->sps.data[2]); /* level */
+    AV_W8(p + 4, 0xff); /* 6 bits reserved (111111) + 2 bits nal size length - 3 (11) */
+    AV_W8(p + 5, 0xe1); /* 3 bits reserved (111) + 5 bits number of sps (00001) */
+    AV_WB16(p + 6, h->sps.data_size + 1);
+    AV_W8(p + 8, NAL_SPS | (3 << 5)); // NAL unit header
+    memcpy(p + 9, h->sps.data, h->sps.data_size);
+    p += 9 + h->sps.data_size;
+    AV_W8(p + 0, 1); /* number of pps */
+    AV_WB16(p + 1, h->pps.data_size + 1);
+    AV_W8(p + 3, NAL_PPS | (3 << 5)); // NAL unit header
+    memcpy(p + 4, h->pps.data, h->pps.data_size);
+
+    p += 4 + h->pps.data_size;
+    av_assert0(p - vt_extradata == vt_extradata_size);
+
+    data = CFDataCreate(kCFAllocatorDefault, vt_extradata, vt_extradata_size);
+    av_free(vt_extradata);
     return data;
 }