Improved subtitiles/OSD quality when the rendering is done by the vout display module.

In this case, the subpictures are rendered either at the display size or
the video size (the biggest one).

src/video_output/video_output.c

@@ -918,7 +918,18 @@ static int ThreadDisplayRenderPicture(vout_thread_t *vout, bool is_forced)
     const vlc_fourcc_t *subpicture_chromas;
     video_format_t fmt_spu;
     if (do_dr_spu) {
-        fmt_spu = vd->source; /* TODO improve */
+        vout_display_place_t place;
+        vout_display_PlacePicture(&place, &vd->source, vd->cfg, false);
+
+        fmt_spu = vd->source;
+        if (fmt_spu.i_width * fmt_spu.i_height < place.width * place.height) {
+            fmt_spu.i_sar_num = vd->cfg->display.sar.num;
+            fmt_spu.i_sar_den = vd->cfg->display.sar.den;
+            fmt_spu.i_width          =
+            fmt_spu.i_visible_width  = place.width;
+            fmt_spu.i_height         =
+            fmt_spu.i_visible_height = place.height;
+        }
         subpicture_chromas = vd->info.subpicture_chromas;
     } else {
         if (do_early_spu) {