Step one, without changing anything in the saved XCFs yet:

Abstract reading and writing of file offsets away into their own
xcf_read_offset() and xcf_write_offset() functions, which take
"goffset" instead of "guint32". Also change xcf_seek_pos() to take a
goffset argument.

Change all file offset variables in xcf-load.c, xcf-write.c and struct
XcfInfo to goffset, and add new member "bytes_per_offset" to XcfInfo,
which is currently always 4.