Summary:
So far the Platform performed a deep query into the AbstractEglBackend
to get information such as EGLContext, EGLConfig, EGLSurface. This
change adjusts this so that the AbstractEGLPlatform forwards it directly
whenever it gets informed about one following the approach already used
for EGLDisplay. This simplifies the code a lot and allows to remove the
dependency on the actual scene backend from the Platform (in order to
split out the SceneOpenGL into a plugin).

Test Plan:
Run nested kwin_wayland, triggered Outline which requires all
those methods.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7698