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.c

@@ -0,0 +1,48 @@
+#include "packet_merger.h"
+
+#include "util/log.h"
+
+void
+sc_packet_merger_init(struct sc_packet_merger *merger) {
+    merger->config = NULL;
+}
+
+void
+sc_packet_merger_destroy(struct sc_packet_merger *merger) {
+    free(merger->config);
+}
+
+bool
+sc_packet_merger_merge(struct sc_packet_merger *merger, AVPacket *packet) {
+    bool is_config = packet->pts == AV_NOPTS_VALUE;
+
+    if (is_config) {
+        free(merger->config);
+
+        merger->config = malloc(packet->size);
+        if (!merger->config) {
+            LOG_OOM();
+            return false;
+        }
+
+        memcpy(merger->config, packet->data, packet->size);
+        merger->config_size = packet->size;
+    } else if (merger->config) {
+        size_t config_size = merger->config_size;
+        size_t media_size = packet->size;
+
+        if (av_grow_packet(packet, config_size)) {
+            LOG_OOM();
+            return false;
+        }
+
+        memmove(packet->data + config_size, packet->data, media_size);
+        memcpy(packet->data, merger->config, config_size);
+
+        free(merger->config);
+        merger->config = NULL;
+        // merger->size is meaningless when merger->config is NULL
+    }
+
+    return true;
+}