ffmpeg: delay processing of subtitles before filters are initialized

If a subtitle packet came before the first video frame could be fully
decoded, the subtitle packet would get discarded. This puts the subtitle
into a queue instead, and processes it once the attached filter graph is
initialized.

ffmpeg.h

@@ -337,6 +337,7 @@ typedef struct InputStream {
     struct sub2video {
         int64_t last_pts;
         int64_t end_pts;
+        AVFifoBuffer *sub_queue;    ///< queue of AVSubtitle* before filter init
         AVFrame *frame;
         int w, h;
     } sub2video;
@@ -636,6 +637,8 @@ int filtergraph_is_simple(FilterGraph *fg);
 int init_simple_filtergraph(InputStream *ist, OutputStream *ost);
 int init_complex_filtergraph(FilterGraph *fg);
 
+void sub2video_update(InputStream *ist, AVSubtitle *sub);
+
 int ifilter_parameters_from_frame(InputFilter *ifilter, const AVFrame *frame);
 
 int ffmpeg_parse_options(int argc, char **argv);