This is a patch over MR !108 by @sandsmark. Martin has done a great job implementing support and I find it a shame not to use it.
I made sure that the results are the same as the current version and fixed the problems of pixels with wrong colors with color depth grater than 8 bits. I also fixed conversion errors on mask and gray images (16/32 bits).
Unfortunately the internal rendering engine of the original code is 8-bit so I always forced the output of 8-bit images to correct the problems (see image below). Since it is a plugin with a "rendering engine", the tests to seriously validate it are potentially endless (as the original version is not perfect).
Errors of the original version of the MR (right) which should no longer occur:
We now support up to and including version 11 of the XCF format, earlier
it only supported version 1 (from 1997, according to the XCF spec).
Biggest difference seems to be that they changed to 64bit for offsets
from version 11 and upwards, otherwise it's mostly just newer enum
values and theoretically major stuff that we don't really need to care
about to get a thumbnail (e. g. linear vs. perceptual RGB).
We still don't support all features, but now it handles that more
gracefully and should at least create thumbnails that are usable. It
should also be easier to update in the future if/when there comes new
versions.
Also added a test file created with the latest version of Gimp
(2.10.18).
Reviewed By: aacid
Differential Revision: https://phabricator.kde.org/D25937
