Avoid massive overdrawing with sequenceIndex != 0
Commit 8ed64be9 ("Enlarge image in folder preview when there's only one image") made createSubThumbnail part of drawSubThumbnail. As a side effect, when skipValidItems is > 0, the thumbnail is not only fetched but also drawn in the top left position. Split fetching and drawing again. This is visible in the file dialogs (dolphin only uses sequenceIndex==0), when hovering a directory with the mouse.
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f1c90489