Incorrect rain vfx

Although rain has long been implemented in OpenMW, it behaves differently to vanilla Morrowind. The rain in OpemMW more obviously "follows" the player's movement, and is most noticeable when moving around. Additionally, the rain in vanilla appears to have several layers at varying distances to give it greater depth. There may also be a slight difference in transparency and size.

(RM-3536 from redmine: created on 2016-09-09 by August Avatar, , closed on 2017-07-05 by nobody, duplicates #2698 (closed))