Refactoring KWin's OpenGL support

Martin Graesslin has posted a detailed account of his recent work on KWin's OpenGL compositor, including separating OpenGL 1.0 and OpenGL 2.0 support, and a refactor that can use EGL as the backend on X, rather than GLX. "The change also nicely grouped all the OpenGL 1 code into one area which will be easier to remove once we decide that the End of Life is reached. Removing code is not as easy as one might expect and that is actually quite a project."