[scenes/opengl] Merge window classes

Summary:
Legacy OpenGL 1 compositing backend had been dropped quite a while ago
so some of OpenGL scene classes can be merged back.

Test Plan: Compiles, windows are rendered as before.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26700