Summary:
Currently when a client changes it's buffer size the result is corrupt
until we move the window.

When a client buffer scale changes we need to redraw the whole window,
and most importantly flush the cached window quads, which reference the
scale.

This is not handled by the doSetGeometry() from the size changing as the
normalised size of the client remains the same so it no-ops.

Test Plan:
Opened gtk3-demo and changed screen scale. It's now perfect.
Patch for Qt to work correctly incoming.

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: graesslin, plasma-devel, kwin, #kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7486