Remove the 1px border from thumbnails

This is part of https://git.reviewboard.kde.org/r/129921/.

Returning DrawFrame in flags results in a 1px black and white border
getting added to the thumbnail while returning None does not.
There is no reason why it should be adding a border.
Especially since this thumbnail creator generates images that could (and
should IMO) be treated the same as thumbnails from, say, image files
like wallpapers etc.

REVIEW: 129923