sub: add --osd-blur and --sub-text-blur options

These require bleeding edge libass (latest git version), and will be
ignored otherwise.

I'm not sure about the blur factor and scaling. The ASS/VSFilter
semantics for blur scaling are a bad mess. Might require further
investigation.

DOCS/man/en/options.rst

@@ -1351,6 +1351,9 @@
 --osd-back-color=<#RRGGBB>, --sub-text-back-color=<#RRGGBB>
     See ``--osd-color``. Color used for OSD/sub text background.
 
+--osd-blur=<0..20.0>, --sub-text-blur=<0..20.0>
+    Gaussian blur factor. 0 means no blur applied (default).
+
 --osd-border-color=<#RRGGBB>, --sub-text-border-color=<#RRGGBB>
     See ``--osd-color``. Color used for the OSD/sub font border.
 

sub/ass_mp.c

@@ -69,6 +69,9 @@ void mp_ass_set_style(ASS_Style *style, struct osd_style_opts *opts)
     style->MarginV = opts->margin_y * scale;
     style->ScaleX = 1.;
     style->ScaleY = 1.;
+#if LIBASS_VERSION >= 0x01020000
+    style->Blur = opts->blur;
+#endif
 }
 
 ASS_Track *mp_ass_default_track(ASS_Library *library, struct MPOpts *opts)

sub/sub.c

@@ -79,6 +79,7 @@ const struct m_sub_options osd_style_conf = {
         OPT_FLOATRANGE("spacing", spacing, 0, -10, 10),
         OPT_INTRANGE("margin-x", margin_x, 0, 0, 300),
         OPT_INTRANGE("margin-y", margin_y, 0, 0, 600),
+        OPT_FLOATRANGE("blur", blur, 0, 0, 20),
         {0}
     },
     .size = sizeof(struct osd_style_opts),

sub/sub.h

@@ -192,6 +192,7 @@ struct osd_style_opts {
     float spacing;
     int margin_x;
     int margin_y;
+    float blur;
 };
 
 extern const struct m_sub_options osd_style_conf;