Avoid fractionals in GridDelegate sizes and alignments

Summary: The thumbnail object within the Delegate is offset by float amounts

Test Plan:
TBH it all looks the same to me.
Gammray no longer has float values for the inner Thumbnail offset

Reviewers: Plasma, ngraham, rooty

Reviewed By: ngraham, rooty

Subscribers: rooty, ngraham, filipf, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19530