sub/sd_ass: set ASS_OVERRIDE_BIT_BLUR if available

libass/libass@4df64d0

Fixes: #14852

DOCS/interface-changes/sub-ass-override-blur.txt

@@ -0,0 +1 @@
+`sub-blur` is now applied by `sub-ass-override=force` with new enough libass

DOCS/man/options.rst

@@ -2545,7 +2545,8 @@ Subtitles
 ``--sub-ass-override=<no|yes|scale|force|strip>``
     Control whether user style overrides should be applied. Note that all of
     these overrides try to be somewhat smart about figuring out whether or not
-    a subtitle is considered a "sign".
+    a subtitle is considered a "sign" and try to be as non-destructive as
+    possible.
 
     :no:    Render subtitles as specified by the subtitle scripts, without
             overrides.
@@ -2554,7 +2555,8 @@ Subtitles
             rendering.
     :scale: Like ``yes``, but also apply ``--sub-scale`` (default).
     :force: Like ``yes``, but also force all ``--sub-*`` options. Can break
-            rendering easily.
+            rendering easily. Certain options aren't overridden if they can
+            potentially be too destructive.
     :strip: Radically strip all ASS tags and styles from the subtitle. This
             is equivalent to the old ``--no-ass`` / ``--no-sub-ass`` options.
 

sub/sd_ass.c

@@ -533,6 +533,9 @@ static void configure_ass(struct sd *sd, struct mp_osd_res *dim,
                             | ASS_OVERRIDE_BIT_COLORS
                             | ASS_OVERRIDE_BIT_BORDER
                             | ASS_OVERRIDE_BIT_SELECTIVE_FONT_SCALE;
+#if LIBASS_VERSION >= 0x01703020
+        set_force_flags |= ASS_OVERRIDE_BIT_BLUR;
+#endif
     }
     if (shared_opts->ass_style_override[sd->order] == ASS_STYLE_OVERRIDE_SCALE)
         set_force_flags |= ASS_OVERRIDE_BIT_SELECTIVE_FONT_SCALE;