basic frame timing logging
This adds basic frame timing logging to the state tracker and to the compositor.
OXR_ST_LOG=trace prints timestamps for when xrWaitFrame is called, when it finishes waiting, the predicted display time and period, as well as when xrEndFrame is called and what predicted display time is passed in xrEndFrame.
XRT_COMPOSITOR_LOG=trace prints time stamps for when the compositor waits, the predicted display time and period it computes, as well as when layers are committed with their respective predicted display times.
Of course this does not replace the need for proper profiling but it gives a very high level overview whether Monado is even attempting to do the right thing (in my tests the output looks good).