Whereas normal layers of dimension 0x0 are definitely broken, group and
text layers depend on their contents, which will be able to resize the
layer appropriately and fix whatever rendering. This commit allows to
salvage such layers, hence make XCF loading even more resistant to
certain form of file corruption.

This commit (and the previous one) are not theoretical but the result of
discovering some old corrupted file, with an empty group of size 0x0
(saved by GIMP itself, because of some old bug). Rather than destroying
these layer groups, this just allows us to reopen them without any kind
of loss!

(cherry picked from commit 06be0746)