avformat/matroskaenc: use display aspect ratio for DisplayWidth and DisplayHeight when possible

This avoids potential rounding errors and guarantees the source aspect
ratio is preserved.
Keep writing pixel values when Stereo 3D Mode is enabled and for WebM,
as the format doesn't support anything else.

This fixes ticket #5743, implementing the suggestion from ticket #5903.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskaenc.c

@@ -1204,8 +1204,19 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
                 return AVERROR(EINVAL);
             }
             if (d_width != par->width || display_width_div != 1 || display_height_div != 1) {
-                put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYWIDTH , d_width / display_width_div);
-                put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYHEIGHT, par->height / display_height_div);
+                if (mkv->mode == MODE_WEBM || display_width_div != 1 || display_height_div != 1) {
+                    put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYWIDTH , d_width / display_width_div);
+                    put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYHEIGHT, par->height / display_height_div);
+                } else {
+                    AVRational display_aspect_ratio;
+                    av_reduce(&display_aspect_ratio.num, &display_aspect_ratio.den,
+                              par->width  * (int64_t)st->sample_aspect_ratio.num,
+                              par->height * (int64_t)st->sample_aspect_ratio.den,
+                              1024 * 1024);
+                    put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYWIDTH,  display_aspect_ratio.num);
+                    put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYHEIGHT, display_aspect_ratio.den);
+                    put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYUNIT, MATROSKA_VIDEO_DISPLAYUNIT_DAR);
+                }
             }
         } else if (display_width_div != 1 || display_height_div != 1) {
             put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYWIDTH , par->width / display_width_div);