better timebase selection heuristic for video stream copy

fixes -ss -t -vcodec copy

Originally committed as revision 6847 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -1535,7 +1535,10 @@ static int av_encode(AVFormatContext **output_files,
             codec->bit_rate = icodec->bit_rate;
             codec->extradata= icodec->extradata;
             codec->extradata_size= icodec->extradata_size;
-            codec->time_base = icodec->time_base;
+            if(av_q2d(icodec->time_base) > av_q2d(ist->st->time_base))
+                codec->time_base = icodec->time_base;
+            else
+                codec->time_base = ist->st->time_base;
             switch(codec->codec_type) {
             case CODEC_TYPE_AUDIO:
                 codec->sample_rate = icodec->sample_rate;