Allow gapless frames

kitty/graphics.h

@@ -57,6 +57,7 @@ typedef struct {
     ImageRef *refs;
     Frame *extra_frames, root_frame;
     uint32_t current_frame_index, frame_id_counter;
+    uint64_t animation_duration;
     size_t refcnt, refcap, extra_framecnt;
     monotonic_t atime;
     size_t used_storage;