vf_drawtext: do not reset the frame number in config_input.

Frame number should be incremented normally even if the link properties
change.

libavfilter/vf_drawtext.c

@@ -569,7 +569,6 @@ static int config_input(AVFilterLink *inlink)
 
     s->var_values[VAR_X] = 0;
     s->var_values[VAR_Y] = 0;
-    s->var_values[VAR_N] = 0;
     s->var_values[VAR_T] = NAN;
 
     av_lfg_init(&s->prng, av_get_random_seed());