Add support for v4l2loopback

It allows to send the video stream to /dev/videoN, so that it can be
captured (like a webcam) by any v4l2-capable tool.

PR #2232 <https://github.com/Genymobile/scrcpy/pull/2232>
PR #2233 <https://github.com/Genymobile/scrcpy/pull/2233>
PR #2268 <https://github.com/Genymobile/scrcpy/pull/2268>

Co-authored-by: Romain Vimont <rom@rom1v.com>

app/src/v4l2_sink.h

@@ -0,0 +1,39 @@
+#ifndef SC_V4L2_SINK_H
+#define SC_V4L2_SINK_H
+
+#include "common.h"
+
+#include "coords.h"
+#include "trait/frame_sink.h"
+#include "video_buffer.h"
+
+#include <libavformat/avformat.h>
+
+struct sc_v4l2_sink {
+    struct sc_frame_sink frame_sink; // frame sink trait
+
+    struct video_buffer vb;
+    AVFormatContext *format_ctx;
+    AVCodecContext *encoder_ctx;
+
+    char *device_name;
+    struct size frame_size;
+
+    sc_thread thread;
+    sc_mutex mutex;
+    sc_cond cond;
+    bool stopped;
+    bool header_written;
+
+    AVFrame *frame;
+    AVPacket packet;
+};
+
+bool
+sc_v4l2_sink_init(struct sc_v4l2_sink *vs, const char *device_name,
+                  struct size frame_size);
+
+void
+sc_v4l2_sink_destroy(struct sc_v4l2_sink *vs);
+
+#endif