FPS counter

[katajakasa]

DO NOT MERGE! This uses GetPerformanceCounter, which is not very portable. We should wait for SDL3 and SDL_GetTicksNS() for this.

So, for dev testing only for now!

[Vagabond]

This seems silly to need SDL3 for, we could just use a platform dependent high resolution timing source, or use a library like https://github.com/nclack/tictoc

[katajakasa]

This seems silly to need SDL3 for, we could just use a platform dependent high resolution timing source, or use a library like https://github.com/nclack/tictoc

We don't need SDL3 for this, but this is not important enough to pull in yet another dependency, therefore we should merge this when we need to swap to SDL3.

[Nopey]

SDL3's SDL_GetTicksNS uses SDL_GetPerformanceCounter, and some values calculated from SDL_GetPerformanceFrequency.

https://github.com/libsdl-org/SDL/blob/5c8bed378040007d0e2e9267ff4bc76c016cee8f/src/timer/SDL_timer.c#L624

What about GetPerformanceCounter is not portable?
We could avoid the (double) arithmetic if we're careful with our Uint64's