osd_libass: update styles when OSD changes PlayRes

The OSD style settings depend on the PlayRes, simply because all style
values are implicitly scaled by the PlayResY in libass. Also, the OSC
changes the PlayResY in certain situations, so something could go wrong.
But not sure if this actually matters in practice.

sub/ass_mp.c

@@ -43,9 +43,11 @@ void mp_ass_set_style(ASS_Style *style, double res_y,
                       const struct osd_style_opts *opts)
 {
     if (opts->font) {
-        free(style->FontName);
-        style->FontName = strdup(opts->font);
-        style->treat_fontname_as_pattern = 1;
+        if (!style->FontName || strcmp(style->FontName, opts->font) != 0) {
+            free(style->FontName);
+            style->FontName = strdup(opts->font);
+            style->treat_fontname_as_pattern = 1;
+        }
     }
 
     // libass_font_size = FontSize * (window_height / res_y)

sub/osd_libass.c

@@ -96,31 +96,36 @@ static void create_ass_track(struct osd_state *osd, struct osd_object *obj,
     track->PlayResY = res_y ? res_y : MP_ASS_FONT_PLAYRESY;
     track->PlayResX = res_x ? res_x : track->PlayResY * aspect;
     track->WrapStyle = 1; // end-of-line wrapping instead of smart wrapping
+    track->Kerning = true;
 
     // Force libass to clear its internal cache - it doesn't check for
     // PlayRes changes itself.
     if (old_res_x != track->PlayResX || old_res_y != track->PlayResY)
         ass_set_frame_size(obj->osd_render, 1, 1);
 
-    if (track->n_styles == 0) {
-        track->Kerning = true;
+    if (track->n_styles < 2) {
         int sid = ass_alloc_style(track);
         track->default_style = sid;
         ASS_Style *style = track->styles + sid;
         style->Alignment = 5; // top-title, left
         style->Name = strdup("OSD");
-        mp_ass_set_style(style, track->PlayResY, osd->opts->osd_style);
         // Set to neutral base direction, as opposed to VSFilter LTR default
         style->Encoding = -1;
 
         sid = ass_alloc_style(track);
+        assert(sid == track->default_style + 1);
         style = track->styles + sid;
         style->Name = strdup("Default");
-        const struct osd_style_opts *def = osd_style_conf.defaults;
-        mp_ass_set_style(style, track->PlayResY, def);
         style->Encoding = -1;
     }
 
+    ASS_Style *s_osd = track->styles + track->default_style;
+    mp_ass_set_style(s_osd, track->PlayResY, osd->opts->osd_style);
+
+    ASS_Style *s_def = track->styles + track->default_style + 1;
+    const struct osd_style_opts *def = osd_style_conf.defaults;
+    mp_ass_set_style(s_def, track->PlayResY, def);
+
     obj->osd_track = track;
 }