cogl/onscreen/glx: Fix x/y mixup used to identify the current output

I haven't seen this cause a problem but it looks like the worst case is that it would have put the wrong refresh rate value in CoglFrameInfo for multi-monitor systems of differing frequencies. But even that seems unlikely given _cogl_xlib_renderer_output_for_rectangle chooses the output with the greatest overlap of the partially-correct rectangle.