These are callbacks which are invoked by VIPS during evaluation. They
are called each time a region is filled with data (for PIO functions), or
each time im_writeline()
is called (for WIO functions). The callback
has access to a large struct containing information about the progress of
evaluation, useful for user-interfaces built on top of VIPS. See the manual
page for im_add_eval_callback()
for full details.