Fixed scrollbar background bug for transparent Konsole

Summary:
In D11184 a visual glitch was introduced when using transparent
background in Konsole.

BUG: 391788
FIXED-IN: 18.04

Test Plan:
Before:

Opaque, there was no problem here

Transparent, obvious problem

After:

Opaque, nothing changes here

Transparent, problem fixed

Reviewers: hindenburg, Konsole

Reviewed By: hindenburg, Konsole

Subscribers: Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D11839