Implment DRM EGL scaling

Summary: We need to set the viewport so that we scale from device pixels to global compositor space.

Test Plan:
Ran kwin_wayland properly on my laptop without setting KWIN_COMPOSE.
Most things worked.

Reviewers: #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3504