[perf] Add glx.swapComplete event

[perf] Add glx.swapComplete event

Add an event when we receive an event on buffer swap completion; we'll
only get this if Clutter is using the INTEL_swap_event GLX extension,
but it's useful to see the actual timing of video frames.

The recorded event includes the actual timestamp of the swap, since
we are given that in the GLX event - on my system it tends to be
consistently 80-100us before we log the event, but if something was
going wrong in event handling (too much synchronous work), then that
could could show up as a longer delay.

https://bugzilla.gnome.org/show_bug.cgi?id=619516