lavfi/ass: use a default DAR value of 1.0

Previously it was using the same value of the input video DAR, which is
inconsistent with most implementations.

Fix trac ticket #1098.

libavfilter/vf_ass.c

@@ -134,7 +134,6 @@ static int config_input(AVFilterLink *inlink)
     const AVPixFmtDescriptor *pix_desc = &av_pix_fmt_descriptors[inlink->format];
     double sar = inlink->sample_aspect_ratio.num ?
         av_q2d(inlink->sample_aspect_ratio) : 1;
-    double dar = inlink->w / (double)inlink->h * sar;
 
     av_image_fill_max_pixsteps(ass->pix_step, NULL, pix_desc);
     ff_fill_rgba_map(ass->rgba_map, inlink->format);
@@ -143,7 +142,7 @@ static int config_input(AVFilterLink *inlink)
     ass->vsub = pix_desc->log2_chroma_h;
 
     ass_set_frame_size  (ass->renderer, inlink->w, inlink->h);
-    ass_set_aspect_ratio(ass->renderer, dar, sar);
+    ass_set_aspect_ratio(ass->renderer, 1.0, sar);
 
     return 0;
 }