kwinglutils: Deprecate GLTexture dirty state tracking

It's currently being used only by the X11 standalone backend. We should
either port the X11 backend to manual dirty state tracking or waiting
until it gets dropped. The main motivation for getting rid of the dirty
state tracking in the GLTexture is that it keeps kwin open for
alternative opengl wrappers, e.g. QOpenGL, and it simplifies GLTexture
code.