osd: make it possible to have different subtitle vs. OSD defaults

Until now, they used exactly the same defaults for the styling options.
The defaults were shared, so it was impossible to have different
defaults. Change this. This requires duplicating the full default
struct, even for settings that are the same. The list of options is
still shared, though.

sub/osd.h

@@ -137,6 +137,7 @@ struct osd_style_opts {
 };
 
 extern const struct m_sub_options osd_style_conf;
+extern const struct m_sub_options sub_style_conf;
 
 struct osd_state;
 struct osd_object;