libavcodec: factor out SEI generation for A53 captions

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/internal.h

@@ -345,4 +345,19 @@ AVCPBProperties *ff_add_cpb_side_data(AVCodecContext *avctx);
 
 int ff_side_data_set_encoder_stats(AVPacket *pkt, int quality, int64_t *error, int error_count, int pict_type);
 
+/**
+ * Check AVFrame for A53 side data and allocate and fill SEI message with A53 info
+ *
+ * @param frame      Raw frame to get A53 side data from
+ * @param prefix_len Number of bytes to allocate before SEI message
+ * @param data       Pointer to a variable to store allocated memory
+ *                   Upon return the variable will hold NULL on error or if frame has no A53 info.
+ *                   Otherwise it will point to prefix_len uninitialized bytes followed by
+ *                   *sei_size SEI message
+ * @param sei_size   Pointer to a variable to store generated SEI message length
+ * @return           Zero on success, negative error code on failure
+ */
+int ff_alloc_a53_sei(const AVFrame *frame, size_t prefix_len,
+                     void **data, size_t *sei_size);
+
 #endif /* AVCODEC_INTERNAL_H */