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.