Bug 730211 - Extra zeroes appear in XCF files

Don't use xcf_seek_end() because that seems to be broken on certain
file systems / operating systems / FUSE mounts / whatever. Instead,
seek to explicitly calculated file offsets.

Ported Massimo's patch to master and added comments --Mitch

The same commit in gimp-2-8 is a57e49b1bbd318b06c4f8d578557b63256a1e66d