Use a chan<- bool to signal to the resizing goroutine that it should
abort its work ... and not touch the destination image anymore.

Additionally <-chan Step is used for results, disallowing closing it
again and making the recover()'s obsolete.

wdeview now works snappier, no data access races anymore and is
easier to reason about.

If not work is ever aborted the done channel can be just discarded.