Capture and print CPU load on a per-frame basis.

Capture and print CPU load on a per-frame basis.

We use clock_gettime to measure the amount of CPU time accrued by the
active process. Then, by subtracting the value between succesive frame
times and dividing by the total wall-clock time for the frame, we
determine a measure of the per-frame CPU load from 0.0 to 1.0.

We now print the frame number, frame time, frame latency, and CPU load
in a single-line format for simpler parsing, (with a header comment
describing the names and units for each field).