As noted in commit ce3f55292a an explict glFlush is needed for
both glBlitFramebuffer and glXCopySubBuffer.
_clutter_backend_glx_blit_sub_buffer was already doing an explicit
flush when using glBlitFramebuffer, so just do it unconditonally
and remove the call from clutter_stage_glx_redraw.
http://bugzilla.clutter-project.org/show_bug.cgi?id=2558