libavfilter/scale2ref: Maintain main input's DAR

The scale2ref filter will now maintain the DAR of the main input and
not the DAR of the reference input. This previous behavior was deemed
counterintuitive for most (all?) use-cases.

Before:
scale2ref=iw/4:ow/mdar
in  w:320 h:240 fmt:rgb24 sar:1/1
ref w:640 h:360 fmt:rgb24 sar:1/1
out w:160 h:120 fmt:rgb24 sar:4/3 flags:0x2
SAR: ((120 * 640) / (160 * 360)) * (1 / 1) = 4 / 3
DAR: (160 / 120) * (4 / 3) = 16 / 9
(main out now same DAR as ref)

Now:
scale2ref=iw/4:ow/mdar
in  w:320 h:240 fmt:rgb24 sar:1/1
ref w:640 h:360 fmt:rgb24 sar:1/1
out w:160 h:120 fmt:rgb24 sar:1/1 flags:0x2
SAR: ((120 * 320) / (160 * 240)) * (1 / 1) = 1 / 1
DAR: (160 / 120) * (1 / 1) = 4 / 3
(main out same DAR as main in)

The scale2ref FATE test has also been updated.
Signed-off-by: Kevin Mark <kmark937@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_scale.c

@@ -337,10 +337,10 @@ static int config_props(AVFilterLink *outlink)
         }
     }
 
-    if (inlink->sample_aspect_ratio.num){
-        outlink->sample_aspect_ratio = av_mul_q((AVRational){outlink->h * inlink->w, outlink->w * inlink->h}, inlink->sample_aspect_ratio);
+    if (inlink0->sample_aspect_ratio.num){
+        outlink->sample_aspect_ratio = av_mul_q((AVRational){outlink->h * inlink0->w, outlink->w * inlink0->h}, inlink0->sample_aspect_ratio);
     } else
-        outlink->sample_aspect_ratio = inlink->sample_aspect_ratio;
+        outlink->sample_aspect_ratio = inlink0->sample_aspect_ratio;
 
     av_log(ctx, AV_LOG_VERBOSE, "w:%d h:%d fmt:%s sar:%d/%d -> w:%d h:%d fmt:%s sar:%d/%d flags:0x%0x\n",
            inlink ->w, inlink ->h, av_get_pix_fmt_name( inlink->format),

tests/ref/fate/filter-scale2ref_keep_aspect

@@ -5,7 +5,7 @@
 #media_type 0: video
 #codec_id 0: rawvideo
 #dimensions 0: 160x120
-#sar 0: 4/3
+#sar 0: 1/1
 #stream#, dts,        pts, duration,     size, hash
 0,          0,          0,        1,    57600, 9a19c23dc3a557786840d0098606d5f1
 0,          1,          1,        1,    57600, e6fbdabaf1bb0d28afc648ed4d27e9f0