The backend is based on drm's gbm backend and also uses the GBM
platform, but with the default display allowing the driver to pick
a device. In addition it doesn't use a surface, but a surfaceless
context with a framebuffer object to render to. Given that it diverged
too much from drm's backend to allow more code sharing.
If KWin is started from a tty it gets a proper driver, if KWin is started
in a X11 session it only gets llvmpipe. Given that there is a chance that
the autotests using the virtual backend will fail. In that case a follow
up patch will enforce either O2 or Q.
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.
