GimpExportOptions is incomplete.
It exists so the API is stable.
For now, ScriptFu eat and ignore actual args,
binding to NULL, in calls to PDB procedures
file-export-foo having formal args of this type.
Unlikely that in the future ScriptFu will do anything else:
if a plugin needs export options, use another language.
The few file exporters used by existing ScriptFu scripts
(file-gbr-export and file-pat-export)
don't honor export options.
cb1a46511a