IMAGE descriptors have a flag called kill which can be used
to block computation. If im->kill is set to a non-zero value,
then any VIPS pipelines which use im as an intermediate will fail
with an error message. This is useful for user-interface writers --
suppose your interface is forced to close an image which many other images
are using as a source of data. You can just set the kill flag in all
of the deleted image's immediate children and prevent any dangling pointers
from being followed.