Cleanup krita thumbnailer
The thumbnailer added a white background instead of preserving any transparency, as every other thumbnailer does (SVG, PNG, ...).
Also remove the scaling of the image, which is already handled in the core thumbnailer code in kio.
Optimize loading of the thumbnails, use KArchiveFile::createDevice()
instead of an intermediate QBuffer QIODevice.
Edited by Stefan Brüns