progs/perf: add first attempt at a swapbuffers rate test

progs/perf: add first attempt at a swapbuffers rate test

This is pretty ugly as the original framework assumed you'd set
a single window size at startup and keep it throughout, but for
swapbuffers you want to test the rate at various window sizes.

With luck a nicer solution can be found, but this at least lays out
a marker.