fps display fix

Use gettime(GETTIME_FRAMESTART) instead of gettime(GETTIME_REALTIME) to calculate qc fps as the latter becomes inaccurate after a while (at least on Windows 10).

It should be noted that gettime(GETTIME_FRAMESTART) returns the value of the engine variable realtime which is used by the engine to calculate the engine fps.

Bug description: after a while the fps value fluctuates between 2 values multiple of 4, after a longer while between 2 values multiple of 8 and so on.

Test with:

hud_panel_engineinfo_framecounter 1
hud_panel_engineinfo_framecounter_exponentialmovingaverage 0