It doesn't make much sense any more as we do no longer link EGL since the
switch to epoxy. And epoxy pulls it in at runtime if needed.
Even more on Wayland it's just plain stupid to have EGL disabled. So
removing the option just simplifies our code base without any
disadvantages.
REVIEW: 124695
Each of the backends becomes a plugin. This allows kwin_wayland to load
the requested plugin and kwin itself doesn't need to link all the
libraries needed. E.g. libdrm is no longer linked if running kwin_x11.
Also this allows to create backends for the non-standard EGL platforms
(examples could be raspberrypi or Android devices).
