Scaled decorations in QPainter mode

Summary:
Under wayland we support high DPI putting by putting a separation
between the logical co-ordinate system and the resolution of rendered
assets.

I didn't include window decorations in the previous wayland scaling
patchset. They were drawn them at a standard resolution, which is
implicitly scaled up.

This uses the Qt scaling, meaning oxygen and breeze (and others) get
perfect high DPI support with zero client changes.

Like the window scaling this handles any combination of a 2x scaled
decoration being rendered on a 1x screen or vice versa.

CCBUG: 384765

Test Plan:
export KWIN_COMPOSE=Q
Had two screens of different scales
It was the right size on both (as before)
Was super-sharp on the fancy screen

Reviewers: Plasma, hetzenecker, graesslin

Reviewed By: Plasma, graesslin

Subscribers: ngraham, graesslin, plasma-devel, kwin, KWin

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8504