Move querying the egl extensions into the AbstractEglBackend

Summary:
Instead of having the egl extensions in the global kwinglutils lib it
becomes private to the AbstractEglBackend. Just like on glx the
glxextensions are moved into the platform.

The extensions are queried from initEglAPI, that is as early as possible
after initializing the EGLDisplay. This ensures that any implementing
subclass can access the extensions early enough.

As a note: the EglOnXBackend had a potentially wrong sequence for
initializing the buffer age extension. It is now moved to the correct
place where the result is needed for the first time.

From the global API eglExtensions are removed from hasGLExtension and
the eglExtensions function is dropped. As by that initEGL did not do
anything it is also dropped.

Test Plan:
Tested nested kwin on Wayland, still works, extensions shown
in debug console

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3396