KWin::GLTexture support external textures

Summary:
Currently KWin::GLTexture is a nice wrapper for creating a GL texture
and performing various operations including rendering.

In a pending patch I want to render the FBO from a QQuickScene directly,
where we have an underlying texture already, but it makes sense to
re-use this class for the rendering. A similar need is in haagch's kwin
VR patchset.

This patch adds a constructor to GLTexture that takes an externally
managed texture and provides the relevant useful KWin features.

Test Plan: Used in patch series

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D24206