avfilter/vf_delogo: unbreak fate

It is not clear what was real intention of previous commit to this filter.
It was not working correctly, hopefully this is fixed now.
It never checked that new x/y/w/h are actually valid, hopeffully this is fixed now.
It uses named variables in expressions that are never set, still not fixed.
It does not set named variables that uses actual frame widht/height, making actual
expressions less usable for our users, still now fixed.

libavfilter/vf_delogo.c

@@ -318,6 +318,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     int direct = 0;
     int plane;
     AVRational sar;
+    int ret;
 
     s->var_values[VAR_N] = inlink->frame_count_out;
     s->var_values[VAR_T] = TS2T(in->pts, inlink->time_base);
@@ -326,6 +327,17 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     s->w = av_expr_eval(s->w_pexpr, s->var_values, s);
     s->h = av_expr_eval(s->h_pexpr, s->var_values, s);
 
+    ret = config_input(inlink);
+    if (ret < 0) {
+        av_frame_free(&in);
+        return ret;
+    }
+
+    s->w += s->band*2;
+    s->h += s->band*2;
+    s->x -= s->band;
+    s->y -= s->band;
+
     if (av_frame_is_writable(in)) {
         direct = 1;
         out = in;