fftools/ffplay: set the alphamodes buffersink array option instead of the binary one

And add the missing goto fail. This should ensure the alpha mode is set and remove
bogus warnings printed by ffplay.

Signed-off-by: James Almer <jamrial@gmail.com>

fftools/ffplay.c

@@ -1938,9 +1938,10 @@ static int configure_video_filters(AVFilterGraph *graph, VideoState *is, const c
                                 AV_OPT_TYPE_INT, sdl_supported_color_spaces)) < 0)
         goto fail;
 
-    if ((ret = av_opt_set_array(filt_out, "alpha_modes", AV_OPT_SEARCH_CHILDREN,
+    if ((ret = av_opt_set_array(filt_out, "alphamodes", AV_OPT_SEARCH_CHILDREN,
                                 0, FF_ARRAY_ELEMS(sdl_supported_alpha_modes),
                                 AV_OPT_TYPE_INT, sdl_supported_alpha_modes)) < 0)
+        goto fail;
 
     ret = avfilter_init_dict(filt_out, NULL);
     if (ret < 0)