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.
Some tests, eg small SubData are probably overwhelmed by the cost of
performing the draw after each upload. Add a varient which does a lot
of subdata uploads and then a single draw.
Also try to avoid cache-artifacts in the upload timings.
