sub: add --ass-style-override=force option

(The old "force" choice of that option is renamed to "force-default".)

This allows overriding native ASS script subtitle styles with the style
provided by the --sub-text-* options (like --sub-text-font etc.). This
is disabled by default, and needs to be explicitly enabled with the
--ass-style-override=force option and input property.

This uses in fact exactly the same options (--sub-text-*) and semantics
as the ones used to configure unstyled text subtitles.

It's recommended to combine this with this in the mpv config file:

ass-force-style="ScaledBorderAndShadow=1"   # work around dumb libass behavior

Also, adding a key binding to toggle this behavior should be added,
because overriding can easily break:

L cycle ass-style-override

This would cycle override behavior on Shift+L and allows quickly
disabling/enabling style overrides.

Note: ASS should be considered a vector format rather than a subtitle
format. There is no easy or reliable way to determine whether the style
of a given subtitle event can be changed without destroying visuals or
not. This patch relies on a simple heuristic, which often works and
often breaks.

DOCS/man/en/options.rst

@@ -110,6 +110,10 @@ OPTIONS
         rendering of ASS/SSA subtitles. It can sometimes be useful to forcibly
         override the styling of ASS subtitles, but should be avoided in general.
 
+    .. note::
+
+        Try using ``--ass-style-override=force`` instead.
+
 ``--ass-force-style=<[Style.]Param=Value[,...]>``
     Override some style or script info parameters.
 
@@ -158,18 +162,21 @@ OPTIONS
 
         Using this option may lead to incorrect subtitle rendering.
 
-``--ass-style-override=<yes|no|force>``
+``--ass-style-override=<yes|no|force|force-default>``
     Control whether user style overrides should be applied.
 
     :yes:   Apply all the ``--ass-*`` style override options. Changing the default
             for any of these options can lead to incorrect subtitle rendering
             (default).
     :no:    Render subtitles as forced by subtitle scripts.
-    :force: Like ``yes``, but also override the style named ``Default`` to
-            make it look like the like text subtitle style implied by the
+    :force-default: Like ``yes``, but also override the style named ``Default``
+            to make it look like the like text subtitle style implied by the
             ``--sub-text-...`` option. This won't always work, because the
             dialogue style doesn't necessary use this name, and it might break
             other advanced uses of the ASS format.
+    :force: Try to force the font style as defined by the ``--sub-text-*``
+            options. Requires a modified libass, can break rendering easily.
+            Probably more reliable than ``force``.
 
 ``--ass-use-margins``
     Enables placing toptitles and subtitles in black borders when they are

options/options.c

@@ -427,7 +427,7 @@ const m_option_t mp_opts[] = {
     OPT_CHOICE("ass-shaper", ass_shaper, 0,
                ({"simple", 0}, {"complex", 1})),
     OPT_CHOICE("ass-style-override", ass_style_override, 0,
-               ({"no", 0}, {"yes", 1}, {"force", 2})),
+               ({"no", 0}, {"yes", 1}, {"force-default", 2}, {"force", 3})),
     OPT_FLAG("osd-bar", osd_bar_visible, 0),
     OPT_FLOATRANGE("osd-bar-align-x", osd_bar_align_x, 0, -1.0, +1.0),
     OPT_FLOATRANGE("osd-bar-align-y", osd_bar_align_y, 0, -1.0, +1.0),

sub/ass_mp.c

@@ -139,6 +139,7 @@ void mp_ass_configure(ASS_Renderer *priv, struct MPOpts *opts,
     float set_line_spacing = 0;
     float set_font_scale = 1;
     int set_hinting = 0;
+    int set_force_override = 0;
     if (opts->ass_style_override) {
         set_use_margins = opts->ass_use_margins;
 #if LIBASS_VERSION >= 0x01010000
@@ -147,6 +148,7 @@ void mp_ass_configure(ASS_Renderer *priv, struct MPOpts *opts,
         set_line_spacing = opts->ass_line_spacing;
         set_font_scale = opts->sub_scale;
         set_hinting = opts->ass_hinting;
+        set_force_override = opts->ass_style_override == 3;
     }
 
     ass_set_use_margins(priv, set_use_margins);
@@ -155,6 +157,13 @@ void mp_ass_configure(ASS_Renderer *priv, struct MPOpts *opts,
 #endif
 #if LIBASS_VERSION >= 0x01000000
     ass_set_shaper(priv, opts->ass_shaper);
+#endif
+#if LIBASS_VERSION >= 0x01103000
+    ass_set_selective_style_override_enabled(priv, set_force_override);
+    ASS_Style style = {0};
+    mp_ass_set_style(&style, 288, opts->sub_text_style);
+    ass_set_selective_style_override(priv, &style);
+    free(style.FontName);
 #endif
     ass_set_font_scale(priv, set_font_scale);
     ass_set_hinting(priv, set_hinting);