Center only during drawing, not the hit rects

Summary:
center only the drawing of the components but never modify the
subcontrolrects (and hit targets)

BUG: 416348
FIXED-IN: 5.18.0

Test Plan:
now the handle activates immediately without one single pixel
margin

Reviewers: VDG, hpereiradacosta, ndavis

Reviewed By: VDG, ndavis

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26783