[GridDelegate] Fix gaps in corners of thumbnailArea highlight
Summary: There was a small gap in the corners of the thumbnailArea highlight rectangle that was noticable on dark thumbnails.
Test Plan:
Before:
After:
Reviewers: Plasma, VDG, filipf
Reviewed By: VDG, filipf
Subscribers: filipf, GB_2, ngraham, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D20140