FrameSvgItem: fix textureRect for tiled subitems to not shrink to 0

Summary:
The old logic results in broken rendering if the sample was bigger than the
area to render, due to normalized texture rect being 0 in the respective
dimansion. As well as stretched mapping of the sample for fractional
relationships between the area to render and the sample size.

Test Plan:
Borders of backgrounds of FluffyBunny theme render properly, no other
regressions seen (but unsure what else might rely on that artifact).

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: apol, sitter, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19787