Add virtual method to Scene to get the EGL/GLX extensions

Summary:
We had a few places (e.g. DebugConsole, Platform) where the Scene was
cased into a SceneOpenGL to access the backend and get the extensions.

This change simplifies that by adding a virtual method to Scene directly
which is implemented in SceneOpenGL and returns the backend's
extensions.

Thus the casts to SceneOpenGL are no longer required.

Test Plan:
Opened debug console to verify extensions are listed,
triggered Outline to verify the sharing QPA context gets created.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7734