sd_ass: introduce sub-ass-prune-delay

Disabled by default because it breaks sub-seek and playback in cases
where the user changes play-dir from + to - during runtime and past
"seen" events need to be re-rendered.

Available since libass/libass@dcc9eb7

DOCS/interface-changes/sub-ass-prune-delay.txt

@@ -0,0 +1,2 @@
+add `sub-ass-prune-delay` option to control libass automatica pruning of past
+events

DOCS/man/options.rst

@@ -2533,6 +2533,22 @@ Subtitles
     ``complex`` is the default. If libass hasn't been compiled against HarfBuzz,
     libass silently reverts to ``simple``.
 
+``--sub-ass-prune-delay=<-1|seconds>``
+    Set the delay for automatic pruning of events from memory in libass. When
+    enabled, subtitle events are removed from memory once their end timestamp is
+    older than the specified delay.
+
+    :-1:        disables automatic pruning (default).
+    :seconds:   specify how many seconds after an event is no longer displayed
+                should the pruning occur. ``0`` prunes events as soon as they're
+                off screen.
+
+    .. note::
+
+        This breaks sub-seek and subtitle rendering when changing play-direction
+        from forward to backward during runtime for events that were already
+        "seen" and need to be rendered again, if those events got pruned.
+
 ``--sub-ass-styles=<filename>``
     Load all SSA/ASS styles found in the specified file and use them for
     rendering text subtitles. The syntax of the file is exactly like the ``[V4

options/options.c

@@ -336,6 +336,7 @@ const struct m_sub_options mp_subtitle_sub_opts = {
             {"none", 0}, {"light", 1}, {"normal", 2}, {"native", 3})},
         {"sub-ass-shaper", OPT_CHOICE(ass_shaper,
             {"simple", 0}, {"complex", 1})},
+        {"sub-ass-prune-delay", OPT_DOUBLE(ass_prune_delay), M_RANGE(-1.0, DBL_MAX)},
         {"sub-ass-justify", OPT_BOOL(ass_justify)},
         {"sub-scale-by-window", OPT_BOOL(sub_scale_by_window)},
         {"sub-scale-with-window", OPT_BOOL(sub_scale_with_window)},
@@ -355,6 +356,7 @@ const struct m_sub_options mp_subtitle_sub_opts = {
         .sub_scale_by_window = true,
         .sub_use_margins = true,
         .sub_scale_with_window = true,
+        .ass_prune_delay = -1.0,
         .teletext_page = 0,
         .sub_scale = 1,
         .ass_vsfilter_color_compat = 1,

options/options.h

@@ -118,6 +118,7 @@ struct mp_subtitle_opts {
     char *ass_styles_file;
     int ass_hinting;
     int ass_shaper;
+    double ass_prune_delay;
     bool ass_justify;
     bool sub_clear_on_seek;
     int teletext_page;

sub/sd_ass.c

@@ -247,6 +247,10 @@ static void assobjects_init(struct sd *sd)
     ass_set_check_readorder(ctx->ass_track, sd->opts->sub_clear_on_seek ? 0 : 1);
 #endif
 
+#if LIBASS_VERSION >= 0x01703010
+    ass_configure_prune(ctx->ass_track, sd->opts->ass_prune_delay * 1000.0);
+#endif
+
     enable_output(sd, true);
 }