Perfstats averaging and refactor

Refactors the perfstats code and adds support for averaging, standard deviation, minimum and maximum.

New cvar ps_samplesize controls the sample size for average/etc calculation. Setting it to 1 (the default value) disables average/etc calculation. Accepted range is 1-1000.

New cvar ps_descriptor changes the statistic that is calculated from the samples. Values:

• 1: Average
• 2: Standard deviation
• 3: Minimum
• 4: Maximum

Addresses some suggestions in #668 (closed).

Test build: (based on next so be careful with your save files)

srb2win_perfstats-averaging-rebased2.exe

(edit 2021-11-10: Updated test build to include newest commit)