… than passing TIFF header offsets around everywhere.

Note that while BigTIFF supports files greater than 2 GiB in size, our current implementation does not due to the pervasive use of Int32 throughout the code to represent offsets into the data.

This will only ever be a 16-bit value.

This allows combining the add and test operations into a single lookup.

If the byte order changes during extraction, a new reader is made via WithByteOrder.
That new reader is naturally scoped to the sub-reading operation, and the byte order
does not need to be reverted.