Fix interpolation related stats in perfstats

chromaticpipe ‎requested to merge
chromaticpipe2/SRB2:fixinterpstats into next

Currently, the way that the stats ps_interp_frac and ps_interp_lag are set up is incorrect, leading to them always displaying 0 instead of the correct value. This is because PS_TIME is meant for values returned from I_GetPreciseTime . This merge request fixes that.

Shoutouts to @Indev for helping me find this out

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