demux: mkv: set the video format SAR based on the DisplayUnit of the track

We can apply the SAR once we know the cropped pixels dimensions and the actual
display dimension requested.

Apply the DAR when DisplayUnit is 3 [1].

[1] https://www.ietf.org/archive/id/draft-ietf-cellar-matroska-08.html#name-displayunit-element

modules/demux/mkv/matroska_segment_parse.cpp

@@ -555,8 +555,25 @@ void matroska_segment_c::ParseTrackEntry( const KaxTrackEntry *m )
                                             (i_crop_left + i_crop_right);
             tk->fmt.video.i_visible_height = tk->fmt.video.i_height -
                                             (i_crop_top + i_crop_bottom);
-            /* FIXME: i_display_* allows you to not only set DAR, but also a zoom factor.
-               we do not support this atm */
+
+            if (i_display_height && i_display_width)
+            {
+                switch (i_display_unit)
+                {
+                    case 0: // pixels
+                    case 1: // centimeters
+                    case 2: // inches
+                        vlc_ureduce( &tk->fmt.video.i_sar_num, &tk->fmt.video.i_sar_den,
+                                     i_display_width * tk->fmt.video.i_visible_height,
+                                     i_display_height * tk->fmt.video.i_visible_width, 0);
+                        break;
+                    case 3: // display aspect ratio
+                        vlc_ureduce( &tk->fmt.video.i_sar_num, &tk->fmt.video.i_sar_den,
+                                     i_display_height * tk->fmt.video.i_visible_width,
+                                     i_display_width * tk->fmt.video.i_visible_height, 0);
+                        break;
+                }
+            }
         }
 #if LIBMATROSKA_VERSION >= 0x010406
         E_CASE( KaxVideoProjection, proj )