Use SDL's high resolution timer

Fixes perfstats only having millisecond precision on platforms other than Winbruh. Perfstats also use timer precision directly, and convert the time difference to microseconds.

I also refactored M_DrawPerfStats...

I_GetTime has a moving epoch. It's probably not necessary but I'm sure now that the time isn't going to wrap around.

There is only one change to the perfstat drawing as a result of the refactor:

(In low resolution, opengl batching calls are drawn to the right of polygons and vertices. They were separated in high resolution too so I thought this made more sense.)