Summary:
with the new very slim scrollbars, the rounded rectangle could
go to have a radius more than half their width, making them look
"pointy", this ensures they stay properly round

Test Plan: slim scrollbars still round, bigger ones are unaltered

Reviewers: #plasma, #vdg, hpereiradacosta, hein

Reviewed By: #plasma, hein

Subscribers: hein, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10080