For GIMP patterns we have maximum allowed dimensions which we check when
loading a pattern. However, we did not check this when saving a pattern.
See issue #6032.
This commit adds a check when saving a pattern and adds a descriptive
error to make clear why saving fails.
(cherry picked from commit f130fe1917)
Add GimpData::save() implementation to GimpPattern, and change some
glue code to make patterns editable.
Also implement GimpData::duplicate() in GimpPatternClipboard, which
makes it possible to simply copy an area and duplicate the clipboard
pattern to create a new persistent pattern.
(cherry picked from commit e93fd73fac)
