avdevice: deprecate sdl outdev

Signed-off-by: J. Dekker <jdek@itanimul.li>

libavdevice/sdl2.c

@@ -51,6 +51,7 @@ typedef struct {
     SDL_Rect texture_rect;
 
     int inited;
+    int warned;
 } SDLContext;
 
 static const struct sdl_texture_format_entry {
@@ -165,6 +166,15 @@ static int sdl2_write_header(AVFormatContext *s)
     int i, ret = 0;
     int flags  = 0;
 
+    if (!sdl->warned) {
+        av_log(sdl, AV_LOG_WARNING,
+            "The sdl output device is deprecated due to being fundamentally incompatible with libavformat API. "
+            "For monitoring purposes in ffmpeg you can output to a file or use pipes and a video player.\n"
+            "Example: ffmpeg -i INPUT -f nut -c:v rawvideo - | ffplay -\n"
+        );
+        sdl->warned = 1;
+    }
+
     if (!sdl->window_title)
         sdl->window_title = av_strdup(s->url);