D21330: Optionally use images' embedded thumbnails for greater performance, even if they're too small

[Nathaniel Graham]

ngraham accepted this revision.
This revision is now accepted and ready to land.

REVISION DETAIL
  https://phabricator.kde.org/D21330

To: tommo, #gwenview, ngraham
Cc: yurchor, kde-doc-english, ngraham, #gwenview, tommo, gennad, clintmoyer, 
tdarboux, huoni, muhlenpfordt, rkflx, skadinna