New image flattening functionality breaks saving functionality

[Robert Nordan]

Thanks to dogfooding myself the git master of Pinta, a discovered a curious flaw: Every PNG image I saved was completely blank, and every JPG was completely black. ORA images saved fine. This led me to believe that an error was introduced in the new blend mode respecting layer flattening introduced with [1]. I tested by reverting to [2] , and it worked correctly. Moving the revisions up to [1] broke it again, so the flaw definitely appears there.

[1] https://github.com/PintaProject/Pinta/commit/f0fddbd10d133ec8ad97e0129511f4e656337474
[2] https://github.com/PintaProject/Pinta/commit/fc2bae95dd7c173c1426acccddae8beeb12e7af9

I think maybe we have to remember to always save a file when we test stuff to make sure it works, since it's a bit silly that this flaw has been in master for 15 days without being noticed. :P

[Cameron White (cameronwhite91)]

I think this was sort of noticed in bug #999571, but that was only for some png images.

[Robert Nordan (rpvn)]

Ah, quite right. Marking this as a dupe and upgrading the other bug.