Let's rather not build the plugin if we don't have the dependency
then building it without OpenGL support. Simplifies the code a bit
and makes the backend overall more useful and goes along with e.g.
the Wayland one which has EGL also as a hard dependency for the
plugin.
REVIEW: 124697
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).
