avcodec/libopenh264enc: Allow user-supplied buffers

Here the packet size is known before allocating the packet,
so that supporting user-supplied buffers is trivial.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/libopenh264enc.c

@@ -30,6 +30,7 @@
 #include "libavutil/mathematics.h"
 
 #include "avcodec.h"
+#include "encode.h"
 #include "internal.h"
 #include "libopenh264.h"
 
@@ -404,10 +405,9 @@ static int svc_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     }
     av_log(avctx, AV_LOG_DEBUG, "%d slices\n", fbi.sLayerInfo[fbi.iLayerNum - 1].iNalCount);
 
-    if ((ret = ff_alloc_packet2(avctx, avpkt, size, size))) {
-        av_log(avctx, AV_LOG_ERROR, "Error getting output packet\n");
+    if ((ret = ff_get_encode_buffer(avctx, avpkt, size, 0)))
         return ret;
-    }
+
     size = 0;
     for (layer = first_layer; layer < fbi.iLayerNum; layer++) {
         memcpy(avpkt->data + size, fbi.sLayerInfo[layer].pBsBuf, layer_size[layer]);
@@ -433,11 +433,11 @@ const AVCodec ff_libopenh264_encoder = {
     .long_name      = NULL_IF_CONFIG_SMALL("OpenH264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
     .type           = AVMEDIA_TYPE_VIDEO,
     .id             = AV_CODEC_ID_H264,
+    .capabilities   = AV_CODEC_CAP_DR1 | AV_CODEC_CAP_OTHER_THREADS,
     .priv_data_size = sizeof(SVCContext),
     .init           = svc_encode_init,
     .encode2        = svc_encode_frame,
     .close          = svc_encode_close,
-    .capabilities   = AV_CODEC_CAP_OTHER_THREADS,
     .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP |
                       FF_CODEC_CAP_AUTO_THREADS,
     .pix_fmts       = (const enum AVPixelFormat[]){ AV_PIX_FMT_YUV420P,