xwayland: Store EGLContext pointer in lastGLContext

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 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"
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>

(cherry picked from commit b656b0aa)
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