avfilter: add comments for duplicate line

comment about the looks like a duplicate line.
but that is used to reason x is expressed from y

Suggested-by: Paul B Mahol
Suggested-by: Michael Niedermayer
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavfilter/vf_crop.c

@@ -262,6 +262,7 @@ static int filter_frame(AVFilterLink *link, AVFrame *frame)
         NAN : frame->pkt_pos;
     s->var_values[VAR_X] = av_expr_eval(s->x_pexpr, s->var_values, NULL);
     s->var_values[VAR_Y] = av_expr_eval(s->y_pexpr, s->var_values, NULL);
+    /* It is necessary if x is expressed from y  */
     s->var_values[VAR_X] = av_expr_eval(s->x_pexpr, s->var_values, NULL);
 
     normalize_double(&s->x, s->var_values[VAR_X]);

libavfilter/vf_drawtext.c

@@ -1390,6 +1390,7 @@ static int draw_text(AVFilterContext *ctx, AVFrame *frame,
 
     s->x = s->var_values[VAR_X] = av_expr_eval(s->x_pexpr, s->var_values, &s->prng);
     s->y = s->var_values[VAR_Y] = av_expr_eval(s->y_pexpr, s->var_values, &s->prng);
+    /* It is necessary if x is expressed from y  */
     s->x = s->var_values[VAR_X] = av_expr_eval(s->x_pexpr, s->var_values, &s->prng);
 
     update_alpha(s);

libavfilter/vf_overlay.c

@@ -149,6 +149,7 @@ static void eval_expr(AVFilterContext *ctx)
 
     s->var_values[VAR_X] = av_expr_eval(s->x_pexpr, s->var_values, NULL);
     s->var_values[VAR_Y] = av_expr_eval(s->y_pexpr, s->var_values, NULL);
+    /* It is necessary if x is expressed from y  */
     s->var_values[VAR_X] = av_expr_eval(s->x_pexpr, s->var_values, NULL);
     s->x = normalize_xy(s->var_values[VAR_X], s->hsub);
     s->y = normalize_xy(s->var_values[VAR_Y], s->vsub);