Add a new GIMP_EXPORT_NEEDS_CROP export capability, which causes
gimp_export_image() to crop the exported image content to the image
bounds; this is useful for formats that support layers, but have no
concept of global image bounds, hence cropping is the only way to
enforce the image bounds.

When showing the export dialog, give an option to either crop the
layers to the image bounds, or to resize the image to fit the
layers.