Add the option to only time over perf events/markers

[baldurk]

This should hopefully improve accuracy by ameliorating errors and also reduce load and likelihood of crashing from timing every drawcall.

Perhaps there should be a way in the UI to then drill down and time per-drawcall within a perf marker should you so wish?

Ideally I'd fix up the timing so it's accurate and reliable even per-drawcall though :).