plug-ins: pass the format's name again in calls to gimp_export_image()

and clean up the formatting of the call and the lines around it. Now
we can check the various (disabled) export options for regressions
again by changing a single line in gimp_export_image().

(cherry picked from commit 03df8c6af4ec4e5d547e9fff46d0710b3c262c58)