Most expected behavior in normal transform is to see the preview,
whereas you usually don't want to see it in corrective mode. In 2.8
actually, it seems like it was not even possible to see the image
preview in corrective mode.

So let's set "show-preview" to these defaults when "direction" property
is updated.  It is still possible to change it manually for any specific
use cases (i.e. you can hide the preview in normal transform, and
oppositely you can show it in corrective transform), but at least now
defaults are sane.