use stream.time_base for subtitles to keep time from input if possible, patch by ogg.k

src/ffmpeg2theora.c

@@ -1217,8 +1217,15 @@ void ff2theora_output(ff2theora this) {
                         ki->gps_denominator = this->framerate_new.den;
                     }
                     else {
-                        ki->gps_numerator = vstream_fps.num;
-                        ki->gps_denominator = vstream_fps.den;
+                        AVStream *stream = this->context->streams[ks->stream_index];
+                        if (stream->time_base.num > 0) {
+                            ki->gps_numerator = stream->time_base.den;
+                            ki->gps_denominator = stream->time_base.num;
+                        }
+                        else {
+                            ki->gps_numerator = vstream_fps.num;
+                            ki->gps_denominator = vstream_fps.den;
+                        }
                     }
                 }
                 ki->granule_shift = 32;