Add recorder

Implement recording in a separate "class".

app/src/recorder.h

@@ -0,0 +1,24 @@
+#ifndef RECORDER_H
+#define RECORDER_H
+
+#include <libavformat/avformat.h>
+#include <SDL2/SDL_stdinc.h>
+
+#include "common.h"
+
+struct recorder {
+    char *filename;
+    AVFormatContext *ctx;
+    struct size declared_frame_size;
+};
+
+SDL_bool recorder_init(struct recorder *recoder, const char *filename,
+                       struct size declared_frame_size);
+void recorder_destroy(struct recorder *recorder);
+
+SDL_bool recorder_open(struct recorder *recorder, AVCodec *input_codec);
+void recorder_close(struct recorder *recorder);
+
+SDL_bool recorder_write(struct recorder *recorder, AVPacket *packet);
+
+#endif