Very basic: all screens have same size and are ordered from left to
right. It's mostly meant to allow easy test cases with multi-screen.
The quick tiling test demonstrates how it's used.
A new backend which doesn't present the rendered output. It uses a
QPainter scene, renders to a QImage but doesn't present it anywhere.
Thus a real virtual backend.
By exporting the environment variable KWIN_WAYLAND_VIRTUAL_SCREENSHOTS
the backend creates a temporary dir, prints the path to std-out and
saves each rendered frame into that directory. Of course with exit it
will be deleted again.