Fix I/O bugs

[danrbailey]

This addresses some issues in the new feature/io branch, one of which was discovered in production, a couple more through AI code review, one that I believe pre-dated the I/O changes.

• An underflow bug - this is the only serious one which affects VDB Points grids with few attributes that would cause the unsigned number of attributes to underflow.
• The background value was incorrectly using ValueT instead of StorageValueT, which caused an issue reading inactive values.
• Fix an issue when reading old VDB files from prior to 2014 (<221)
• Instance grids are being clipped with the wrong transform, the world-space bbox transform needs to be transformed to the index-space of the instanced grid.