ffmpeg: Make find_stream_info behave like a normal per-file option

Currently it would essentially change the find_stream_info setting for
the file it was specified for and all following files, which is unusual
and somewhat unexpected behaviour for a per-file option and not even
documented to behave like this.

Signed-off-by: Anton Khirnov <anton@khirnov.net>

fftools/ffmpeg.h

@@ -126,6 +126,7 @@ typedef struct OptionsContext {
     int accurate_seek;
     int thread_queue_size;
     int input_sync_ref;
+    int find_stream_info;
 
     SpecifierOpt *ts_scale;
     int        nb_ts_scale;