Replace P_AproxDistance with FixedHypot, and replace FixedHypot's algorithm with R_PointToDist2's

Port of KartKrew/Kart-Public!243 (merged) that also replaces every instance of P_AproxDistance in the code, instead of turning it into a macro for FixedHypot. When P_AproxDistance is used in Lua, generates a deprecated warning to use FixedHypot instead!

Inspired by the work seen in !1232 (merged) and !1309 (merged).