RangeErrors with large images

[PaulHax]

Test image: f32-1024x256.nii.gz (7MB, 1024×1024×256 Float32, 1GB raw)

Bug 1: Save fails silently

1. Load the test image
2. Create a segmentation, paint some voxels
3. Save the session → nothing happens (error only in F12 Console: RangeError: Invalid array length)

Bug 2: Session load fails

1. Load the test image, create a segmentation
2. Configure save format to nii.gz (via config JSON or code change)
3. Save the session, then reload the saved session.volview.zip
4. RangeError: Start offset -N is outside the bounds of the buffer