ffmpeg: better timebase guessing for stream copy

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -2237,13 +2237,23 @@ static int transcode(AVFormatContext **output_files,
                 goto fail;
             memcpy(codec->extradata, icodec->extradata, icodec->extradata_size);
             codec->extradata_size= icodec->extradata_size;
+
+            codec->time_base = ist->st->time_base;
+            if(!strcmp(os->oformat->name, "avi")) {
+                if(!copy_tb && av_q2d(icodec->time_base)*icodec->ticks_per_frame > 2*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;
+                    codec->time_base.den *= 2;
+                }
+            } else if(!(os->oformat->flags & AVFMT_VARIABLE_FPS)) {
                 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,
                         codec->time_base.num, codec->time_base.den, INT_MAX);
-            }else
-                codec->time_base = ist->st->time_base;
+
             switch(codec->codec_type) {
             case AVMEDIA_TYPE_AUDIO:
                 if(audio_volume != 256) {