Keep at least 1 second of frame timings.

This is necessary for 2 reasons - a real one and a fun one.

First, with the difference in monitor refresh rates, we can have 48Hz
latops as well as 240Hz high refresh rate monitors. That's a factor of
4, and tracking frame rates in both situations reliably is kind of hard
- either we track over too many frames and the fps take a lot of time to
adjust, or we track too little time and the fps fluctuate wildly.

Second, when benchmarking with GDK_DEBUG=no-vsync with a somewhat fast
renderer (*cough*Vulkan*cough*) frame rates can go into insane dimensions
and only very few frames are actually getting presentation times
reported. So to report accurate frame rates in those cases, we need a
*very* large history that can be 1000s of times larger than the usual
history. And that's just a waste for normal usage.