thumbnail: better support non-shm case. i.e Windows
This fix testThumbnail for Windows
Requires frameworks/kio!1408 (merged)
Tested with setting WITH_SHM 0 and WITH_SHM 1 in previewjob.cpp and running kio-extras test thumbnailtest for both case. Test pass.
Before the test would fail.
Edited by Méven Car