The purpose of this flag is to have some procedure arguments for which
you wish to ignore last values, or restored values. This may be needed
for arguments which are really volatile and likely won't survive a
session (or even from a run to another).
This will be used in my next commit.
Note: this is very close to GIMP_CONFIG_PARAM_IGNORE, except that this
latter is used for obsolete properties instead, so I felt that it may
not have been the best idea to mix these semantically different flag.
Also GIMP_CONFIG_PARAM_IGNORE properties are not serialized but they are
deserialized, which is not exactly what we want (in most case, it would
work the same, but it also means that if last-used values were to
contain some deprecated value for a property for which we added this
flag, there would be at least one run where a buggy behavior would
happen).
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