Display patch 2

- disable the backing pixmap and double-buffering, we get a better
  result doing this ourselves

- fix up vips reference counting within imagearea

- use the vips_sink_screen() mask image to suppress painting of
  uncalculated pixels

  this fixes the nasty black "flash" we used to get when a toggle on the
  right was changed

- remove the invalidate stuff again, vips-7.39+ does not need this

  unless you use git master vips you'll see black pixels

  add a couple of vips_image_invalidate_all() calls for earlier vipses