The NULL terminator of the tile-offset array of dummy buffer-levels
is erroneously written as an int32, instead of an offset, even in
version-11+ XCFs, in which offsets are 64-bit.

Since the dummy levels aren't actually used by GIMP, we're going to
keep these fields as int32 as an exception, in order to remain
consistent with existing XCFs, and just add a comment in the code,
and update the docs.  If we ever make use of the higher buffer
levels, we should change these fields to offsets, and bump the XCF
version.