The vips driver program was parsing options in a single pass. This
failed if an option came in two parts, for example:
vips --plugin x.plg list
the argument to --plug would be picked up as the action for "vips",
since actions were selected before option parsing
Now we parse in two passes: the first pass picks up options for vips
itself and for the libvips library, then we select the action, then we
parse again, including any options created by the action
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