VideoStore::open() reserves a 64 KiB free box between moov and the first
fragment, but only when the MOV muxer wrote the moov in the header and
moof+mdat fragments follow it. finalize() scans the fragments and fills the
region with free padding followed by a sidx that ends at the first moof, so
FFmpeg-based players (Chromium, Android WebView, Electron) treat the index as
complete and start playback without visiting every fragment. On any parse or
write failure the region stays a free box.
fixes#5144
