ffmpeg: replace "flush Media" with "flush_media" in benchmark_all output

Simplify parsing and consistency.

ffmpeg.c

@@ -1711,11 +1711,11 @@ static void flush_encoders(void)
             switch (enc->codec_type) {
             case AVMEDIA_TYPE_AUDIO:
                 encode = avcodec_encode_audio2;
-                desc   = "Audio";
+                desc   = "audio";
                 break;
             case AVMEDIA_TYPE_VIDEO:
                 encode = avcodec_encode_video2;
-                desc   = "Video";
+                desc   = "video";
                 break;
             default:
                 stop_encoding = 1;
@@ -1731,7 +1731,7 @@ static void flush_encoders(void)
 
                 update_benchmark(NULL);
                 ret = encode(enc, &pkt, NULL, &got_packet);
-                update_benchmark("flush %s %d.%d", desc, ost->file_index, ost->index);
+                update_benchmark("flush_%s %d.%d", desc, ost->file_index, ost->index);
                 if (ret < 0) {
                     av_log(NULL, AV_LOG_FATAL, "%s encoding failed: %s\n",
                            desc,