Set scale on glScissor calls

Summary:
glScissor works on window co-ordinates. i.e not scaled by the viewport
We need to multiply by the scale there.

This fixes a minor visual glitch in animations when using hardware
clipping.

Test Plan:
Hardcoded my minimise animation to be really slow.
Now every frame looks perfect

Reviewers: #kwin

Subscribers: kpiwowarski, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8490