Extract packet merging

Config packets must be prepended to the next media packet. Extract the
logic to a new sc_packet_merger helper to simplify the demuxer code.

app/src/packet_merger.h

@@ -0,0 +1,43 @@
+#ifndef SC_PACKET_MERGER_H
+#define SC_PACKET_MERGER_H
+
+#include "common.h"
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <libavcodec/avcodec.h>
+
+/**
+ * Config packets (containing the SPS/PPS) are sent in-band. A new config
+ * packet is sent whenever a new encoding session is started (on start and on
+ * device orientation change).
+ *
+ * Every time a config packet is received, it must be sent alone (for recorder
+ * extradata), then concatenated to the next media packet (for correct decoding
+ * and recording).
+ *
+ * This helper reads every input packet and modifies each media packet which
+ * immediately follows a config packet to prepend the config packet payload.
+ */
+
+struct sc_packet_merger {
+    uint8_t *config;
+    size_t config_size;
+};
+
+void
+sc_packet_merger_init(struct sc_packet_merger *merger);
+
+void
+sc_packet_merger_destroy(struct sc_packet_merger *merger);
+
+/**
+ * If the packet is a config packet, then keep its data for later.
+ * Otherwise (if the packet is a media packet), then if a config packet is
+ * pending, prepend the config packet to this packet (so the packet is
+ * modified!).
+ */
+bool
+sc_packet_merger_merge(struct sc_packet_merger *merger, AVPacket *packet);
+
+#endif