lavfi/separatefields: adjust tb to avoid rounding errors.

Since the PTS need double accuracy, the timebase denominator is doubled
as well, and this new time base is used for the timestamp unit.

libavfilter/vf_separatefields.c

@@ -41,11 +41,13 @@ static int config_props_output(AVFilterLink *outlink)
         return AVERROR_INVALIDDATA;
     }
 
+    outlink->time_base.num = inlink->time_base.num;
+    outlink->time_base.den = inlink->time_base.den * 2;
     outlink->frame_rate.num = inlink->frame_rate.num * 2;
     outlink->frame_rate.den = inlink->frame_rate.den;
     outlink->w = inlink->w;
     outlink->h = inlink->h / 2;
-    sf->ts_unit = av_q2d(av_inv_q(av_mul_q(outlink->frame_rate, inlink->time_base)));
+    sf->ts_unit = av_q2d(av_inv_q(av_mul_q(outlink->frame_rate, outlink->time_base)));
 
     return 0;
 }