We were storing the pointer to struct glamor_context. However, glamor itself is storing the EGLContext pointer since the commit below. Since the two values could never be equal, this resulted in constant superfluous calls eglMakeCurrent calls. The implicit glFlush triggered by those couldn't be good for performance.
Fixes: 7c88977d "glamor: Store the actual EGL/GLX context pointer in lastGLContext"