[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