Bug 28180: Correctly guess if images have been loaded

Bug 28180: Correctly guess if images have been loaded

window.load won't assure us that the images loaded using JS code are
correctly loaded.

This patch adds a flag to the different providers when the images are
fully loaded.

Note that the img.naturalHeight check is not working for google jackets
and is actually not needed (test with a bib record with and without
google jacket and confirm that it's working as expected).

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>