avconv: make copy_tb on by default.

I.e. on streamcopy set output codec timebase from input stream timebase
(as opposed to input codec timebase). This should be more sane, because
since the stream is not decoded, the input codec tb has no relation to
the timestamps of the copied packets.

avconv.c

@@ -113,7 +113,7 @@ static int video_sync_method= -1;
 static int audio_sync_method= 0;
 static float audio_drift_threshold= 0.1;
 static int copy_ts= 0;
-static int copy_tb;
+static int copy_tb = 1;
 static int opt_shortest = 0;
 static char *vstats_filename;
 static FILE *vstats_file;
@@ -2067,9 +2067,7 @@ static int transcode_init(OutputFile *output_files,
             }
             memcpy(codec->extradata, icodec->extradata, icodec->extradata_size);
             codec->extradata_size = icodec->extradata_size;
-            if (!copy_tb &&
-                av_q2d(icodec->time_base)*icodec->ticks_per_frame > av_q2d(ist->st->time_base) &&
-                av_q2d(ist->st->time_base) < 1.0/500) {
+            if (!copy_tb) {
                 codec->time_base      = icodec->time_base;
                 codec->time_base.num *= icodec->ticks_per_frame;
                 av_reduce(&codec->time_base.num, &codec->time_base.den,