Update threshold to detect real timebase for stream copy.

This may fix a bug where quicktime files use 1/600 timebase.

ffmpeg.c

@@ -2147,7 +2147,7 @@ static int transcode(AVFormatContext **output_files,
                 goto fail;
             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) < 1.0/1000){
+            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){
                 codec->time_base = icodec->time_base;
                 codec->time_base.num *= icodec->ticks_per_frame;
                 av_reduce(&codec->time_base.num, &codec->time_base.den,