Set wayland output scale
Summary:
Provides a virtual method in Screens where backends can supply the scale
of each screen, this is then set on each output.
For the X windowed backend this value is taken from a command line
parameter.
Test Plan:
Ran windowed mode with --scale 1 and 2
then kate --platform=wayland from another screen.
On the latter case UI elements were scaled up correctly
Reviewers: Plasma
Subscribers: plasma-devel, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D3159