fftools/ffmpeg_filter: fix the flags parsing for scaler

Scaler relys on "-sws_flags" option to pass the flags to swscale
and scale filter. Currently passing "sws_flags=xxx" as a filter
option to scaler leads to an incorrect option parsing.

Check and change the string to "flags=xxx" and dumped flags information.
(Refer to parse_sws_flags())

CMD:
$ffmpeg -v verbose -i input.mp4 -sws_flags lanczos+bitexact+accurate_rnd \
        -vf format=yuv420p,scale=800x600 -an -vframes 10 -f md5 -

Before:
[auto_scaler_0 @ 0x7f96c3808680] w:iw h:ih flags:'' interl:0
[auto_scaler_0 @ 0x7f96c3808680] w:1920 h:1080 fmt:yuvj420p sar:0/1 -> w:1920 h:1080 fmt:yuv420p sar:0/1 flags:0x0
[Parsed_scale_1 @ 0x7f96c3806e40] w:1920 h:1080 fmt:yuv420p sar:0/1 -> w:800 h:600 fmt:yuv420p sar:0/1 flags:0x0
MD5=ff1d6091690c6fcd36d458d2a9f648ce

After:
[auto_scaler_0 @ 0x7fe94563b4c0] w:iw h:ih flags:'lanczos+bitexact+accurate_rnd' interl:0
[auto_scaler_0 @ 0x7fe94563b4c0] w:1920 h:1080 fmt:yuvj420p sar:0/1 -> w:1920 h:1080 fmt:yuv420p sar:0/1 flags:0xc0200
[Parsed_scale_1 @ 0x7fe945639d00] w:1920 h:1080 fmt:yuv420p sar:0/1 -> w:800 h:600 fmt:yuv420p sar:0/1 flags:0xc0200
MD5=ff1d6091690c6fcd36d458d2a9f648ce

Signed-off-by: Linjie Fu <linjie.justin.fu@gmail.com>

fftools/ffmpeg_filter.c

@@ -977,7 +977,11 @@ int configure_filtergraph(FilterGraph *fg)
         }
         if (strlen(args))
             args[strlen(args)-1] = 0;
-        fg->graph->scale_sws_opts = av_strdup(args);
+
+        if (!strncmp(args, "sws_flags=", 10)) {
+            // keep the 'flags=' part
+            fg->graph->scale_sws_opts = av_strdup(args+4);
+        }
 
         args[0] = 0;
         while ((e = av_dict_get(ost->swr_opts, "", e,