Summary:
Despite plasma frameworks doing it's own scaling with fonts, it's been
requested to use kwin/wayland scaling.

Like DRM, when kscreen is not used, scale value is loaded from a config
file.

Config format is
[HWComposerOutputs][0]
Scale=N

The 0 is to map similarly to DRM and support multi-screen, but with a
screen index
rather than a UUID based on EDID.

Because we don't support multi screen this is always 0 for now.

Test Plan: Ran with the config value unset and with the config value at
Scale=3.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: bshah, romangg, nicolasfella, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18810