Fix compatibility with old FFmpeg

V4L2 sink used a "url" field format AVFormatContext which has been
introduced in lavf 58.7.100.

Fixes #2382 <https://github.com/Genymobile/scrcpy/issues/2382>

Refs <ea3672b7d6>
Refs <fa8308d3d4>

app/src/compat.h

@@ -22,6 +22,18 @@
 # define SCRCPY_LAVF_REQUIRES_REGISTER_ALL
 #endif
 
+
+// In ffmpeg/doc/APIchanges:
+// 2018-01-28 - ea3672b7d6 - lavf 58.7.100 - avformat.h
+//   Deprecate AVFormatContext filename field which had limited length, use the
+//   new dynamically allocated url field instead.
+//
+// 2018-01-28 - ea3672b7d6 - lavf 58.7.100 - avformat.h
+//   Add url field to AVFormatContext and add ff_format_set_url helper function.
+#if LIBAVFORMAT_VERSION_INT >= AV_VERSION_INT(58, 7, 100)
+# define SCRCPY_LAVF_HAS_AVFORMATCONTEXT_URL
+#endif
+
 #if SDL_VERSION_ATLEAST(2, 0, 5)
 // <https://wiki.libsdl.org/SDL_HINT_MOUSE_FOCUS_CLICKTHROUGH>
 # define SCRCPY_SDL_HAS_HINT_MOUSE_FOCUS_CLICKTHROUGH

app/src/v4l2_sink.c

@@ -180,7 +180,12 @@ sc_v4l2_sink_open(struct sc_v4l2_sink *vs) {
     // still expects a pointer-to-non-const (it has not be updated accordingly)
     // <https://github.com/FFmpeg/FFmpeg/commit/0694d8702421e7aff1340038559c438b61bb30dd>
     vs->format_ctx->oformat = (AVOutputFormat *) format;
-    vs->format_ctx->url = strdup(vs->device_name);
+#ifdef SCRCPY_LAVF_HAS_AVFORMATCONTEXT_URL
+# define CTX_URL_FIELD url
+#else
+# define CTX_URL_FIELD filename
+#endif
+    vs->format_ctx->CTX_URL_FIELD = strdup(vs->device_name);
     if (!vs->format_ctx->url) {
         LOGE("Could not strdup v4l2 device name");
         goto error_avformat_free_context;