This patch creates a GimpExportOptions class in both
libgimpbase and in libgimp. Currently it is a mostly empty
object, but it will be added to after 3.0 to allow for
additional export options (like resizing on export while
leaving the original image intact)
libgimp/gimpexport.c was removed, and most of its content
was copied into libgimp/gimpexportoptions.c. gimp_export_image ()
was replaced with gimp_export_options_get_image () in all
export plug-ins.
GimpExportProcedure has a new function to set the default
image capabilities for each plug-in on creation. It also sets up
a new callback function, which allows the options to respond to
user setting changes (such as toggling 'Save as Animation' in the
GIF or WEBP Plug-in).
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