FrameSvgItem: fix textureRect for tiled subitems to not shrink to 0
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.
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
Differential Revision: https://phabricator.kde.org/D19787