Fix division by zero on g_stageThresholdHalfLife = 0. Values <= 0 disable exponential decrease.
Round stage thresholds.
Rewrite G_CalculateAvgPlayers.

The previous implementation wasn't exact in the first iterations after the first
player joined a team and was run with a high frequency to compensate for this.
Merge branch 'gameplay/viech' into bots+viech