improved image sequence detection heuristic

io/media.cpp

@@ -9,6 +9,10 @@ extern "C" {
 Media::Media() : ready(false) {}
 
 Media::~Media() {
+    reset();
+}
+
+void Media::reset() {
     for (int i=0;i<video_tracks.size();i++) {
         delete video_tracks.at(i);
     }
@@ -17,6 +21,7 @@ Media::~Media() {
     }
     video_tracks.clear();
     audio_tracks.clear();
+    ready = false;
 }
 
 long Media::get_length_in_frames(double frame_rate) {

io/media.h

@@ -37,8 +37,10 @@ struct Media {
     QVector<MediaStream*> audio_tracks;
     int save_id;
     bool ready;
+
     long get_length_in_frames(double frame_rate);
     MediaStream* get_stream_from_file_index(int index);
+    void reset();
 };
 
 int guess_layout_from_channels(int channel_count);