sub: move subtitle FPS adjustment to sd_ass.c

I feel like it's better there. Note that there is no reduced
functionality, as bitmaps subs (i.e. not handled by sd_ass.c) were never
fully read on init, and thus never went through sub_read_all_packets().

On the other hand, this might lead to confusion, as --sub-fps etc. will
now also affect muxed subtitles (which makes not much sense).

sub/sd.h

@@ -22,6 +22,8 @@ struct sd {
     char *extradata;
     int extradata_len;
 
+    struct sh_stream *sh;
+
     // Set to !=NULL if the input packets are being converted from another
     // format.
     const char *converted_from;
@@ -30,6 +32,8 @@ struct sd {
     // the resolution of the VO, nor does it have to be the OSD resolution.
     int sub_video_w, sub_video_h;
 
+    double video_fps;
+
     // Shared renderer for ASS - done to avoid reloading embedded fonts.
     struct ass_library *ass_library;
     struct ass_renderer *ass_renderer;