sub: make use of libavcodec subtitle converters

This allows using some formats that were not supported until now, like
WebVTT.

We still prefer the internal subtitle reader (subreader.c), because
1. Libav, and 2. random things which we probably want to keep, such as
control over formatting, codepage stuff, or various mysterious
postprecessing done in that code.

sub/sd.h

@@ -44,6 +44,8 @@ struct sd_functions {
     void (*reset)(struct sd *sd);
     void (*uninit)(struct sd *sd);
 
+    void (*fix_events)(struct sd *sd);
+
     // decoder
     void (*get_bitmaps)(struct sd *sd, struct mp_osd_res dim, double pts,
                         struct sub_bitmaps *res);