All backends already have an init method so far called from the ctor.
This change moves the call to init out of the OpenGL backends and makes
it the responsibility of the creating code to also call init on the
backend.
This change makes it easier to have virtual methods being called during
the initialization.
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.
