avfilter/drawtext: fix box sizing

At present, box size is clipped to frame size before being drawn,
which can lead to the box not fully covering animated text which is
longer than one or both frame dimensions.

Since ff_blend_rectangle correctly takes care of clipping, it is skipped
here which results in correct box sizing

libavfilter/vf_drawtext.c

@@ -1407,8 +1407,8 @@ static int draw_text(AVFilterContext *ctx, AVFrame *frame,
     update_color_with_alpha(s, &bordercolor, s->bordercolor);
     update_color_with_alpha(s, &boxcolor   , s->boxcolor   );
 
-    box_w = FFMIN(width - 1 , max_text_line_w);
-    box_h = FFMIN(height - 1, y + s->max_glyph_h);
+    box_w = max_text_line_w;
+    box_h = y + s->max_glyph_h;
 
     if (s->fix_bounds) {