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.
As we don't have GLPLatform before the backend is fully created
the AbstractEglBackend has a new method isOpenGLES() -> bool
which determines based on QOpenGLContext::openGLModuleType().
The aim is to be able to create a plugin for each of the backends.
The following directories are created:
* backends/drm
* backends/fbdev
* backends/wayland
* backends/x11
