Add stream layer

The decoder initially read from the socket, decoded the video and sent
the decoded frames to the screen:

              +---------+      +----------+
  socket ---> | decoder | ---> |  screen  |
              +---------+      +----------+

The design was simple, but the decoder had several responsabilities.

Then we added the recording feature, so we added a recorder, which
reused the packets received from the socket managed by the decoder:

                                    +----------+
                               ---> |  screen  |
              +---------+     /     +----------+
  socket ---> | decoder | ----
              +---------+     \     +----------+
                               ---> | recorder |
                                    +----------+

This lack of separation of concerns now have concrete implications: we
could not (properly) disable the decoder/display to only record the
video.

Therefore, split the decoder to extract the stream:

                                    +----------+      +----------+
                               ---> | decoder  | ---> |  screen  |
              +---------+     /     +----------+      +----------+
  socket ---> | stream  | ----
              +---------+     \     +----------+
                               ---> | recorder |
                                    +----------+

This will allow to record the stream without decoding the video.

app/src/stream.h

@@ -0,0 +1,35 @@
+#ifndef STREAM_H
+#define STREAM_H
+
+#include <SDL2/SDL_stdinc.h>
+#include <SDL2/SDL_thread.h>
+
+#include "net.h"
+
+struct video_buffer;
+
+struct frame_meta {
+    Uint64 pts;
+    struct frame_meta *next;
+};
+
+struct stream {
+    socket_t socket;
+    struct video_buffer *video_buffer;
+    SDL_Thread *thread;
+    struct decoder *decoder;
+    struct recorder *recorder;
+    struct receiver_state {
+        // meta (in order) for frames not consumed yet
+        struct frame_meta *frame_meta_queue;
+        size_t remaining; // remaining bytes to receive for the current frame
+    } receiver_state;
+};
+
+void stream_init(struct stream *stream, socket_t socket,
+                 struct decoder *decoder, struct recorder *recorder);
+SDL_bool stream_start(struct stream *stream);
+void stream_stop(struct stream *stream);
+void stream_join(struct stream *stream);
+
+#endif